What this mean?
Your Dungeon Instance is over 24 hours old. You may Reset the Instance if you wish and start at the beginning.

Each dungeon has a beginning and a boss at the end, once you clear it you are sent back to the map and can't enter the dungeon again for 24 hours.

IF you take more than 24 hours you get that message giving you the option to reset the dungeon and start from the beginning