What does it mean when you've been "mentioned" on a forum post?

It means someone has pinged you, for example Its used to get a user to come to a forum post. Or if you have been mentioned in a group eg. Death Note (Ignore this guys)

I must be slow... but pinged me?

Yes, a ping is when a user does '@' plus the name of the user - would be you - to get their attention that they might want to return to the topic of discussion.

Alright. That kind of makes sense now... Thank you both very much.

If you have any more questions you can comment me :)