An Infiltration FOB is when you invade another player's FOB (Forward Operating Base). These became available once you have completed mission 22, which is essentially a tutorial for this type of mission.
Currently getting this to work can be pretty janky. You first need to get your own FOB, but the game has been known to freeze when you purchase it (during the tutorial). So what you do is load the game up in offline mode, turn online mode on from the pause menu, then purchase an FOB. This tends to get around it.
After this, go to the Aerial Command Center and open up the Missions tab on your iDroid. Pick FOB Missions, and choose a base to invade. The achievement pops as soon as you commence the mission, so no pressure to win (they're actually pretty hard).
As of the writing of this guide, these bases default to people your rank. It's entirely possible you could find an empty FoB to attack.