Why can you compress a gas to fit into a smaller space?

Gas molecules generally tend to be very spread apart, and not compacted tightly together (like solids). That means you can "bring" the molecules closer together by compressing them into a smaller space.

If you don't understand, think about a train. If 2 strangers got on an empty carriage, they would probably sit pretty far apart (maybe even opposite ends of the carriage). This doesn't mean that 2 people can fit in the carriage only if they sit at opposite ends though - you can bring those 2 people together and make them sit next to each other, and they'd take up much less space.