A clipboard manager is a tool which allows you to keep a trace of what you’re copying and pasting. Is is really useful when you go through tons of documentation and you want to keep around a bunch of functions you might want to use, for example. The clipboard manager will store an history of everything you do, so that you can get back to older copies you now want to paste.
If you never tried using one, download and install one and give it a try for a few days. Chances are high that you’ll immediately get addicted.