Repeatable quest or more accurate terms for this game, recurring quest, is a kind of quests you can do it repeatedly after fixed hours or days. You may get special tokens, reputations, blesses and etc. depending on what quest you are doing.
I'd like to give some catalogs and examples, but the posts above are already in details. So I just give some add-ons here:
1. The symbol before the quest name indicates its catalog. You'll see the symbol when you talk with npc to accept the quest. For most of the recurring quests it's in green, some are in blue (elder bless, rage/rainbow etc). (Recently admins added some recurring quests for high level, they are in yellow as normal quest but repeatable.)
2. In quests tab, there's a recurring section, after you finish a quest, if it's a recurring quest and still available, it will appear there, and you can also see when you can do it again.