Target Focus is going to do exactly as its name suggests: it's going to first move the Focal Plane (area that is in focus) onto the Subject you have targeted, bringing your Subject into a clear, sharp focus. Once there, it's then going to lock the Focal Plane to that same Subject which gives us two interesting results;
If you move the camera, the subject will stay in focus.
If you move the subject, the subject will still stay in focus.
In essence, this is auto-focus and is found in pretty much every DSLR/mirror-less modern camera on the market. Even for the hobbyist photographers among you, this should be a fairly well-understood concept.
First, start by selecting your subject. This can either be a Player character in Multiplayer, an Actor (such as an NPC), a specific bone in a Static Mesh, or a Location itself.
By enabling Focus on Target, we lock the Focal Plane to the selected Subject.
Disabling Focus on Target removes the lock, and the subject will no longer remain in focus.
To enable Focus on Target while using your gamepad, first select a target then press down the LThumbstick while in the Alternate Control scheme.