Text inside the <!-- .... --> tag is recognized as HTML comment.

You may use special <!--UdmComment--> .... <!--/UdmComment--> comment tags to exclude the text between from indexing. This may be useful to hide such things like menus and others from indexing.

You may also use <NOINDEX> ... </NOINDEX> as a synonyms to <!--UdmComment--> and <!--/UdmComment-->