What do these symbols on the left next to the threads mean? There so many different symbols...

Fire: Hot topic, probably means theres a lot of activity in this thread.
Lightbulb: Stickied topic
Padlock: Locked topic, a mod locked the topic for some reason, they will often say why they locked in inside.
An I: These are the rules and announcements that are in a seperate square above the normal list of topics.

The square changes as well.

The central square
Black: Unread posts are inside.
Grey: All posts have been read.

The outline
Grey: You haven't posted in this topic yet.
Dark grey: You have posted in this topic before.

Hope this helps, i may have gotten a few things wrong.

SCP-500 wrote:Fire: Hot topic, probably means theres a lot of activity in this thread.
Lightbulb: Stickied topic
Padlock: Locked topic, a mod locked the topic for some reason, they will often say why they locked in inside.
An I: These are the rules and announcements that are in a seperate square above the normal list of topics.

The square changes as well.

The central square
Black: Unread posts are inside.
Grey: All posts have been read.

The outline
Grey: You haven't posted in this topic yet.
Dark grey: You have posted in this topic before.

Hope this helps, i may have gotten a few things wrong.

Thank you, it makes sense to me

Sorry for doubleposting but I noticed that there sometimes is an inner and an outer square. I think only at the board index. What is it?

It means theres a subboard inside

Such as in-game entity discussion in collaboration, and old threads in bug reports.

And you're welcome!.