The clipboard, when referring to computer terminology, is a virtual place where different types of media is stored, temporarily, to enable inter-application communication.
Typically, the clipboard is involved in Cut and Paste, when a user Cuts some text from one application (say, a Web Browser), and pastes it in another (say, a word processor). When the original text is Cut, it is stored on the "virtual" clipboard, and when it is pasted, it is retrieved from the "virtual" clipboard into the new application.
The clipboard acts as an intermediary, and is available to any application on the system. Typically the clipboard contains only a single type of media at a time, and Copying or Cutting another selection will replace the current Clipboard contents.