Ultrapeers are people with a fast connection & whose FWs aren't firewalled. You connect to them & they in turn connect to other UPs & leafs...that's how your searches work.
Ultrapeers will handle most of the search traffic & shield leafs (people with a slower connection or who might be firewalled) from unnecessary traffic...so a leaf only sees searches that relate to the files they're sharing.

If you want to know more about this, use the forum search to look for ultrapeer. Otherwise, you don't have to worry about it