What is a first call resolution? What its impact on calls volume?

This is a call centre terminology, FCR or First Call Resolution.
Sometimes when you call into a call centre to report a problem, a call centre agent generally takes down your problem andgives you a issue ticket number with a promise of resolving your problem in XX hours.
Sometimes when the problem is not resolved in XX hrs, customers generally tend to call back again to cross check whether there issue is resolved or not.
However, at times the agent is able to resolve customer's problem while on the call and hence giving a First Call Resolution.
Higher is the FCR higher is the C-Sat in normal circumstances.

First Call Resolution (FCR) - To fullfil the customer's needs, in an excellent manner, so they won't call a second time for the same topic.
The call volume will be cut in half, as long as the customer doesn't call a second time.