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 usefull to hide such things like menus and others from indexing.
You may also use <NOINDEX> ... </NOINDEX> as a synonims to <!--UdmComment--> and <!--/UdmComment-->