Kingdoms of Amalur: Re-Reckoning

"Locations" are the places that show up on the world map and can be used to fast travel after you've reached them by foot. This includes caves, towns, cities, some camps, all dungeons/separately instanced areas, castles, ruins, etc. You do not have to enter most locations to discover them (especially ones that load you into a separate area), you should get a pop-up on screen that you've discovered the location simply by getting close enough to the entrance/area.

It is recommended to swing by and unlock any you see on your minimap to make travelling the world easier, even if you aren't going to go into that location right away, which will also make progress towards this achievement.

There are over 100 locations in the game (even before the Fatesworn DLC) so if you are regularly completing sidequests (which frequently take you into a cave, castle, ruin, or the like) you should unlock this naturally as you progress.