Means your watching that thread (recieving notifications from it)
Terraria Community Forums
LuciousTPK
Jill
Yep, what Luc said. The Alerts system isn't infallible though, so if you're surprised a thread you like hasn't been getting any Alerts, do check on it.
LuciousTPK
Well inless your me, then a thread you like is probally going to become the next ghost town XD