Prefetching is what windows does in order to run or start applications faster when their icon is clicked either from the shortcut or from the .exe file itself.
Just goto Run and type prefetch and give ok . the prefetch folder will open.
There is no harm in emptying the prefetch folder ,windows will rebuild it again when necessary.You can empty this folder when windows is strangely taking more time to boot.
But never ever delete the prefetch folder itself this might lead to consequences.