Guests are people on the site, viewing the site but not registered and logged in as a member.
Guests has nothing to do with spam. I would guess that most of the site traffic is from guests. Example: a person does a Google search on a related hunting topic and views an HNI thread. That would be a guest viewing the site.
I read as a guest for several years prior to making an account. I was young and wasn't allowed to have online accounts. But could read all the hunting stuff I could find. Learned allot here as a guest before joining and becoming active on the youth forum initially.
-Jake