What exactly is "pinning" in relation to indexes? When one talks about "pinning" in indexes, what exactly is this? Is there some other word/term I can search for, as google has not provided any solutions.

It is part of a test question, where students were asked to define and describe pinning in relation to indexes.

Does anyone have an idea?
Search for Keep Pool.  You're probably better off not using it unless you know its purpose and limitations very well and it fits the problem you are trying to solve.