That is basically a table that holds referer information (what link people clicked on to get to your forum). Since you have so many rows in there, the little pruning tool we have might timeout, but should be able to delete them after being run a few times. If it won't delete any, you might try using something like phpmyadmin, if you have access to it, to empty the table.
We need to get an auto prune setup for that for version 8.
