For this achievement, you need to have all planets 100% explored. If you check your map, the top left will show the percentage for the whole planet, and the bottom left will will show you the percentage for the particular region you are in.
If you are going for the other achievement to collect all secrets and chests, you will get this most of the way done. If you are ever having trouble finishing off a certain region, search for a guide that shows you all of the locations of the force echoes, and that will show you some of the hidden corners of the maps that you've probably overlooked.
I finished this achievement at the top of Witches Horn
*** Spoiler - click to reveal ***during the story, you climb to the top and go into an airborne fight with the Bat boss. Climb back up there and you'll find a room with a chest straight ahead up the stairs, and then a path off to the right that leads downward.