Same as cash. If you visit a webpage your PC will put the pics etc from that site into a cache - basically it will store the pics and pages so if you return to that page within a certain time it will load the pics from your PC which is quicker than pulling them down from the website again
Actually, cache is a general term for a short term/fast access memory holding
recently accessed data. This is designed to speed up repeat access to the same data.
The term cache can be applied to a whole load of things, including processors, and the internet, as iwbus mentions above.