It is an in memory storage, stored in RAM memory of a client machine temporarily. When user request for a same page 2nd time ,the data is displayed from the cache memory ie from RAM memory.
Data retrieving from a repository can be quite a “heavy” task from a performance point of view, especially when the data repository is located far from the application server (e.g., web service call, RPC call, Remoting etc.) or some specific data is accessed very often. So, in order to reduce the workload and time for data retrieving, you can use a caching functionality.