A Web Address is a systems location within the Web. Every system in the Net has a location in the Web where a large tear exists. If a person could stabilize it into a portal it will lead to that system.
When the Saucy Mare was in the Web it traveled to Mainframe's web address in order to return to their home. They were only able to find the web address because they already knew where Mainframe is located. The ship was attacked by Mouse's traps because of its web shields. The traps thought the ship was a web creature. When Bob removed the shields the traps stopped attacking and they were able to stabilize the tear and return home.