It is a tool that lets you perform queries on the contents of a CVS archive; you can: get a list of checkins, see what checkins have been made by a given person, or on a given CVS branch, or in a particular time period. It also includes tools for looking at checkin logs (and comments); doing diffs between various versions of a file; and finding out which person is responsible for changing a particular line of code ("cvsblame").
Here are some example queries that will give you a taste of what Bonsai is capable of:
That's a small sample, but it should be enough to get you started. Play around with the various options on the main Bonsai Query page. If you're updating our cvs tree you'll need to learn about Hacking with Bonsai.
The source code to the Bonsai tool itself is also available, check out the information available at the Bonsai project page.