This mission is called "Eye in the sky" and the goal is to find and steal a very valuable car for a billionaire. In the helicopter, Trevor has access to a camera that can identify individuals on the ground. Talking to Franklin, it's explained as facial recognition, then the in-game HUD states it's a body scan (and it visualizes a body scan of sorts when used), and the Wiki states it's RFID (radio frequency based on electromagnetic fields).
After successfully identifying Chad Mulligan, they follow him with the helicopter until he's lost/hiding in a building. Franklin mentions that Trevor can turn on the thermal vision to find Chad. The gameplay here is highlighting heat signatures and sending Franklin over to them to investigate if they're Chad or not.