This refers to the temporary storage of web pages and other content on the internet in locations other than where they originate. The mechanism is used to lower traffic on the internet and is very effective at speeding up loading of websites that, for instance, contain images that do not change very often. Virtually all web browsers have a local cache on the user's computer that stores all content, that is not explicitly marked as not cacheable, for a brief period of time. Sometimes a proxy server through which users access web pages also cache information, so successive calls to a given web page will load it from this server rather than the originating server that might be much further away.
In Internet advertising, the cache of pages in a cache server or the user's computer means that some ad views won't be known by the ad counting programs; this is a source of concern. There are several techniques for telling the browser not to cache particular pages (see meta tags). On the other hand, specifying no caching for all pages may mean that users will find your site to be slower than you would like.