The sign of the beaver is all over The Sign of the Beaver, and you'll definitely want to hop on over to the "Symbolism, Imagery, Allegory" section to check out a detailed analysis of it. As far as the title is concerned, though, it's pretty much a shout-out to the fact for our main man Matt, the Beaver clan is what this story's all about.
The sign of the beaver represents the clan that Attean and Saknis come from. So while it is literally a simple drawing found on trees, as a title it references how important getting to know Attean and Saknis is for Matt. It is through getting to know these two people—particularly Attean—that Matt really comes into his own, developing into a more confident and competent woodsman, and a less racist person in general.
Really, though—click on over to the "Symbolism" section to really sink your teeth into this title. It's chockfull of meaning.