Compressed sensing (CS) is a way to get lots of information from a small amount of data. Here's a simple example. Imagine you want to take a picture of a big rock. With compressed sensing, you can take a picture with much less data than if you were just using your camera normally - but you will still get the same information!
To do this, special math is used to figure out what pieces of the picture are important enough to keep, and then the rest of the data that isn't as important is thrown away. This way, you don't have to take a picture that has a bunch of extra bits of data that you don't really need, which takes up more space and takes more time.
At the end, you're left with a much smaller picture with all the important information that you would have gotten from the big picture.