This is the official manual for the latest Org-mode release.
Lines starting with zero or more whitespace characters followed by one ‘#’ and a whitespace are treated as comments and, as such, are not exported.
Likewise, regions surrounded by ‘#+BEGIN_COMMENT’ ... ‘#+END_COMMENT’ are not exported.
Finally, a ‘COMMENT’ keyword at the beginning of an entry, but after any other keyword or priority cookie, comments out the entire subtree. In this case, the subtree is not exported and no code block within it is executed either1. The command below helps changing the comment status of a headline.