A browser fingerprint is a collection of info about a browser. Like fingerprints, they are usually unique. With your browser fingerprint governments and advertising companies can track you across the web, making a list of the things you've looked at, the places you've been, and what you've said and done. Linkability is the ability to link two or more profiles, identities, or pieces of data; for example, imagine you have a Gmail account and a Facebook account. You have them registered under different names, and think nobody knows they are both yours, but in reality the advertisers and government agencies have linked you to both of them by using your browser fingerprint to track you across the web. The Tor Browser Bundle helps to prevent this kind of attack by making its fingerprint the same as everyone else's, among other things, thus making it more difficult to track you, identify you, and link your various identities to one person.
For the record, all of this info and more is available at https://www.torproject.org and https://trac.torproject.org/projects/tor/wiki