The more a word is used as a tag, the larger it gets. By selecting a word from the tag cloud, it will show profiles that have been tagged with that word. It is another way to search profiles.
After you select a word from the "Parent Profile Tags," the list below the tag cloud will refine your search even more. For example, if you select "texas" and view 10 profiles, then select "christian" from the list below the tag cloud it will now only show parent profiles that are in Texas AND are Christians.