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-->
For compatibility with ASPSeek, <!--noindex--> ... <!--/noindex--> are also equivalent to <!--UdmComment--> and <!--/UdmComment-->
Google's special comments <!-- google_ad_section_start -->, <!-- google_ad_section_start(weight=ignore) --> and <!-- google_ad_section_end --> consider as tags to include/exclude content for indexing.