There are lots of places on a computer where cache is used.
Generally it is an area where data or information is held so it can be accessed quickly.
Lets look at a simple non-technial example.
Suppose you often sit in front of the TV eating your dinner.
If you want the tomato ketchup you have to get up, go to the kitchen, get the ketchup, take it back to the sofa and sit down again.
Suppose a few minutes later you want the salt. Again you have to get up to go to the kitchen to get the salt.
If you got fed up with this you might put the ketchup, salt, pepper etc on a small table beside the sofa. Then if you want any of them you just reach over and get them rather than having to go to the kitchen each time.
So the area next to the sofa with the ketchup, salt etc is like cache, something you use regulalry and keep nearby.
Computers have areas of cache where regularly used data or computer instructions are stored to speed things up.