A full list of Exploration Challenges can be found in the game’s menu (Challenges > Exploration Challenges). While the other challenges require you to do specific things, the Exploration Challenges are more to do with completing actual content in the game.

Exploration Challenges include the completion of Lost & Found Weapon Quests, Lost & Found Missing Persons Quests, uncovering regions and opening lockboxes, as well as collecting Skill Cards and Journals. You'll get the vast majority of these naturally, especially if you're going for LA Influential (100G).

There are 7 Exploration Challenges to complete, finish the first tier and this achievement is yours.