8.4 Completion
You can often use a feature called completion to help enter arguments. This means that after you type part of the argument, Emacs can fill in the rest, or some of it, based on what was typed so far.
When completion is available, certain keys (usually TAB,
RET, and SPC) are rebound in the minibuffer to special
completion commands (see Completion Commands). These commands
attempt to complete the text in the minibuffer, based on a set of
completion alternatives provided by the command that requested
the argument. You can usually type ? to see a list of
completion alternatives.
Although completion is usually done in the minibuffer, the feature is sometimes available in ordinary buffers too. See Completion for Symbol Names.