What’s a Cache? According to wikipedia,
A cache is a hardware or software component that stores data so future requests for that data can be served faster; the data stored in a cache might be the result of an earlier computation, or the duplicate of data stored elsewhere.
In short, Cache is a technique to store common used resources and return it immediately when someone requests that same resource. These resources are mostly stored in memory. It has significant improvement on the performance of the system with small increase in the storage size.