Marking targets is a new feature added in Borderlands 3. It is very useful when you want to plan a strategy for attacking a stronghold or something like that, since it tracks the enemies on your mini-map, as well as giving them a red glow on your screen. It's especially useful for when you play with other people and want to plan ahead. It will allow you and your group to mark the enemies that you should focus on killing first, which is very nice if you got a sniper in the group.
So, how exactly do we mark targets in Borderlands 3? It's very simple, and the button is the same for both PS4 and Xbox One: Press up on the D-Pad while zooming in with your weapon.
And that's all there is to marking targets.