Observation is a skill that determines how far you map the dungeon you are in with every step you take. You can always see areas you have already explored/mapped. Unmappable/changing dungeons have to be re-explored every visit.
Observation is the easiest skill to train, and likely the first you'll get to 1000. Simply walk around dungeons, something you'll be doing anyway. You only have a small chance to train it when you expose new tiles. Walking back and forth in areas you've already explored doesn't work.