Having never heard of the term guitar forums, I can only guess at the meaning. A forum is like a meeting of the minds, and guitar is something most people know is a musical instrument. So, my interpretation of that term would be an open discussion about guitars. From checking out the Internet, it appears that definition is right on target.

I just Googled the term and found that there are forums of all kinds available to you. All you need to do is look for them. Just regarding guitars, Google came up  with about nine million five hundred and sixty thousand sites. You can break it down even further by designating the type of guitar you want to discuss such as acoustic, electric and so forth.