"In computing, a cache is a hardware or software component that stores data so that future requests for that data can be served faster; the data stored in a cache might be the result of an earlier computation or a copy of data stored elsewhere."
This means that a computer, browser and site will all save images and files so that everything will load faster. This is usually a good thing, but when certain changes are made sometimes they don't reflect right away because of the cache. Clearing it occasionally, especially after updating files or making design changes, will let the system save and see the changes right away.
Below are examples for clearing the cache and cookies on different browsers:
With Safari, the Develop Menu needs to be enabled first.