This achievement requires you to fully explore each area on all planets. In your map the top left shows the percentage for the entire planet while each area has its own individual percentage (on the bottom left as you look at the different areas). Collecting all the secrets, chest, and force echos will help you explore each region but does not guarantee a 100% exploration.
You don't need to collect any of the secrets, force echoes, chests, etc. to obtain the achievement. I have found a guide the shows the maps fully explored so that you can compare your maps to see what you are missing. Also, shows the locations of the various collectibles in each location.
All credit goes to the creator of this website. I didn't create this. I only happened to find it and thought it would be worth sharing.https://game-maps.com/SWJFO/Jedi-Fallen-Order-Maps.asp
Edit 1/29/21-Thanks to StockierHeart- Entries are numbered and can be seen in the databanks to help narrow down missing entries. Planet view explored status may be inaccurate.