The clipboard is just some memory in the computer.
If you ever 'select some text' then 'copy' (or 'cut') and move to some other place in a document e-mail and 'paste' it in, the clipboard (memory) is the 'place' which held the text in the middle while you decided where to paste it back into the item.
If you hit the Print Screen button, then use the 'Start' menu and find MS Paint (assuming you are using Windows) you can then 'paste' the screen copy from the clipboard into Paint as a new image.
Many applications have the File / Edit / ... list of menus. The Edit menu normally has a few standard entries like 'copy' 'paste' 'cut' and 'select all' with keyboard shortcuts like Ctrl+C for copy, Ctrl+V for paste, Ctrl-X for Cut, and Ctrl+A for Select All
You will find those key combinations work in things like Word, Notepad, Wordpad, as well as e-mail software and other applications.
Using the right button on a mouse can sometimes get those operations shown on a menu, too, but if you know the keyboard shortcuts they can sometimes be faster for regular typists or heavy computer users, as the keys are in known positions and you don't have to concentrate on positioning a mouse pointer to choose the desired task.
Sorry to waffle, but in some cases if a mouse stops working, using the keyboard shortcuts can get you out of a jam, and with far too many years of using computers, I should know! :)