The lighting.

[Mentat]

This is probably going to sound like a really stupid question, but I would like to know what it means when the "PF" sign (beside the name of one of the forums) is lit up.

[Kerrie]

the light bulb that is lit up means that new messages have been posted since you last visited...i believe...

[Ben-CS]

http://physicsforums.com/images/on.gif means that there are some new messages in the forum. http://physicsforums.com/images/off.gif means that there are not any new messages in the forum.

[RuroumiKenshin]

Are there any other things that tell you stuff like that?

[STAii]

Yes, there are couple of things.
Under each post you can also see the little "PF" icon either lite or not.
A lite little "PF" icon under a certain post means that the poster is online, an unlite little "PF" icon means the user is offline.
Also under each post (but on the right side this time, IOW under the nickname, the country and those stuff), there is a little leaf-like icon.
The icon can either be yellow (kind of lite) or gray.
When the icon is yellow, it means that you never read this post before.
When the icon is gray, it means that the post was read before (by you).
Edit :
Here is the little leaf i was talking about
http://www.physicsforums.com/images/posticonnew.gif, you can find the date and time of the post next to it.

[Mentat]

Thanks for all the help.