"Paste" is a front end token that inserts the current contents of the Clipboard at the insertion point.
This token is equivalent to the menu command Edit Paste.
Information contained in the Clipboard remains there until a new selection is copied or cut. This lets you paste the same contents repeatedly. You can empty the Clipboard automatically by using the front end token "PasteDiscard".
To execute a front end token, you typically evaluate a command of the form FrontEndExecute[FrontEndToken[token]].