Mass is a measure of how much of something there is. It's not the same as volume, because volume gives an idea of how much space something takes up. It's also different from weight.
If an object has more particles in it, it will have a greater mass. If it has fewer particles, it will have a smaller mass. This means that the mass depends on the thing itself - it does not depend on gravity, so it will have the same mass regardless of where it is. Even on the moon, if you don't change the thing itself, it will have the same mass.
That's all you really need to know about mass. However, you may come across two other concepts related to mass.
Another way that scientists sometimes think about mass is to consider how much something is affected by gravity; the more of something there is, the more it will be affected by gravity (or even create gravity around itself!). This is gravitational mass. If you pick up a small foam ball, it doesn't seem to be affected much by gravity. If you pick up a heavy stone, you can easily feel gravity pulling it down! This is because there are more particles in the stone than in the ball - the stone has a bigger mass. Note, it is different from weight; we're still talking about how much of something there is, not the force of gravity on it.
A further way of thinking about mass is inertial mass. This is where you think about how much you need to push something in order to make it move (or to make it stop if it's already moving). The more of something there is, the more you need to push it to make it move; this is what we mean by inertial mass. Think of this small foam ball rolling down the hill - you can probably stop that with your foot. Now think of the heavy stone rolling down the hill - you need to apply a much bigger force to stop that from moving! That's because the heavy stone has a bigger mass than the ball.
Mass is often expressed in terms of kilogrammes (kg) or, if its really small, grammes (or g).
People often talk about weight when they mean mass. In practice, they end up being very similar and you might be able to get away with using them to mean the same. However, though the two are very easily confused, there is a very basic difference between them. Basically, they're talking about different things.
Mass is talking about how much of something there is - how many particles are brought together to make up a single thing. Weight, on the other hand, is talking about a force - in fact, it's talking about how much force gravity is having on an object. Because it's a force, weight should be measured in Newtons. Obviously if something has a bigger mass, it's going to have a bigger weight. But while mass depends on the thing itself, weight depends on the thing and on gravity.
Let's take an example. Imagine you have one blob. Let's say it has a mass of 9kg (it's quite a heavy blob!).
On earth, that blob will have a weight of roughly 90 Newtons.
However, on the moon, even though the mass of the blob is the same, the effect of gravity is less. Gravity on the moon is about 1/6 times the gravity on earth. This means that instead of a weight of 90 Newtons, the weight of the blob on the moon will only be about 15 Newtons.
The reason we often mix up the two is because gravity is roughly the same anywhere on earth. There are some changes depending on how low down or high up you are; but roughly speaking, gravity is going to be the same. This means that the weight of something is usually affected mostly by its mass.
However, the two things are different. Mass is talking about how much of something there is. Weight is talking about the force of gravity on that thing.
Volume is a way of measuring how much space something takes up. It's different from mass, which tells you how much of something there is, and it's different from density.
Because volume is a three-dimensional concept (that is, volume is an amount of space occupied in height, width and depth - rather than just a flat object on a page), it is measured in cubic metres (m3). However, it can also be measured in terms of a special unit of measurement for volume, such as litres (l).
Density is a way of describing how tightly packed together something is. It gives an indication of how far apart the particles which make something up are spaced. Because something with more crammed into it will weigh more, it has a higher density - a higher mass in a particular volume.
Take for example that classic question: which is heavier, a tonne of feathers or a tonne of stone? Your quick answer is to say 'a tonne of stone', because you know that stone weighs more than feathers. Or does it? In fact, if you get a tonne of feathers, it will weigh the same as a tonne of stone: a tonne!
A better question is to think about density. Density is a way of explaining how much mass there is in a particular volume, and it is simply worked out as mass divided by volume. Density is usually given in terms of mass per volume, for instance: kilograms per cubic metre (kg/m3).
So the real question is, which is more dense, a tonne of feathers or a tonne of stone? In that case, the answer is a tonne of stone. In stone, the particles are packed closer together, so if you have a certain volume of stone, it will weigh more than the same volume of feathers.
Let's think of another example. Let's take our 9kg blob again. This time, theres load of blobs, and they're packed into two containers. One container is bigger than the other, so even though there's the same number of blobs in each, the density is different between the two.
Next, we take look at a particular volume in each container. Let's look at a volume of 3m3 in each one.
In the first container, that volume contains 27 blobs - a total mass of 243kg. If there is a total mass of 243kg in every 3m3, that means the container has a density of 81kg per m3.
In the second container, that volume contains only 1 blob - a total mass of 9kg. If there is a total mass of 9kg in every 3m3, that means the container has a density of 3kg per m3.
So, the more dense something is, the closer packed together it will be. Similarly, the more dense something is, the greater its mass will be for any given volume. Density is a really important topic in science, because often it's not the mass which matters, but how closely packed everything is.