Exact match means that your keywords are exactly what someone types into Google search. For example, you might have the keywords, "Fun iPhone Apps" in your hub. Then Someone's search would be exact if they typed those exact terms in.
However, broad matches would include keyword phrases like:
"Enjoyable iPhone Applications"
"Apps for iPhone that are fun"
"Cool and Fun iPhone Apps"
These mean pretty much the same thing, and Google is smart enough to send you traffic even if your searchers don't use the exact words you have in your hub.