It is a count of known robots that are actively indexing the site. I don't know how long UBB Threads keeps them in the count once they've been detected but it is likely there are at least several hundred actively crawling at any given time.
The bots account for the bulk posts that generate responses/posts. They fall under the, "have you ever type of stuff". Lots of dumb schitt. All fluff and no stuff. Just content and bulk that generates advertising revenue. After all isant everything done for the almighty dollar! Pure money making by the forum, it has nothing to do with quality just quantity. $$$$$