Cache is basically a component used for storing data transparently for fast processing of requests. Data stored in cache can be the computed values or duplicates of original values stored somewhere else. If data is stored in the cache then requests are served simply by reading the cache which is comparatively faster. This tutorial explains you how you can enable cache for Gravatars of your WordPress site.
Caching helps you to speed up your WordPress site and hence gives better user experience. Actually caching creates copy of webpages as static ones, then saves it on server including cached database queries and when your site visitors requests page they get cached page from the server. This reduces the use of resources and makes pages to load fast.