It means an instance of an object has been put into some persistent store. This could be, at the simplest level, something as basic as a Collection.
Consider this class:
This just wraps a Collection as a singleton (so we know there is only one instance of the cache in a program that uses it). In this case the object has been cached "in memory".
There are a multitude of cache implementations for different scenarios (for more complex situations have a google for JCS, EHCache, JBossCache JCache etc. ). All are configurable. A typical cache will have methods to specify how long an object can live in a cache before it becomes stale, what to do once the cache becomes full, how to uniquely identify objects in the cache, how often to check the cache for stale objects and so on. They may also include levels of caching, perhaps an in meneory cache with a File based cache that older objects are moved to once the in memory cache becomes full.