The blue ones are regular threads, the red ones are threads that are popular/have a lot of replies, and the grey ones I believe are the ones in the quarantine room (though I could be wrong) and you need to ask a mod to let you see them.
When there's a green arrow on the thread picture it means you have one or more replies to the thread, and when there's a padlock type thing on it it means that the thread is closed and nobody can reply to it anymore. Hope this helped!
P.S. if I misunderstood any of them or mis-explained them, feel free to correct me.
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