The Explorer Achievement
Complete all optional missions.
Within the game there are optional missions you can complete. Complete them all for the achievement. Not every chapter has them. There are a total of 10 missions, 3 of them which are optional. You can choose to play the optional missions in chapter select if you have a second person in your party. The missions usually consist of getting to a container with parts and circuits.
Click HERE for a guide containing all collectibles and side mission details.