"CompleteSelection" is a front end token that completes a partially typed function name.
This token is equivalent to the menu command Input Complete Selection.
This command reads your current selection (which should be a single word or an insertion point following a word) and looks up possible completions in the list of all currently defined functions and variables. See Complete Selection for more information.
To execute a front end token, you typically evaluate a command of the form FrontEndExecute[FrontEndToken[token]].