comment-dwim is an interactive compiled Lisp function in `newcomment.el'.

It is bound to M-;.

(comment-dwim ARG)

Call the comment command you want (Do What I Mean).
If the region is active and `transient-mark-mode' is on, call
`comment-region' (unless it only consists of comments, in which
case it calls `uncomment-region').
Else, if the current line is empty, call `comment-insert-comment-function'
if it is defined, otherwise insert a comment and indent it.
Else if a prefix ARG is specified, call `comment-kill'.
Else, call `comment-indent'.
You can configure `comment-style' to change the way regions are commented.