Situational awareness is being aware of your surroundings, no matter what situation you find yourself in. Because of your natural fight or flight reflex, your senses are heightened whenever you are put into a stressful situation. Your hearing and vision become sharper. Your mental ability to discern between potentially harmful and neutral factors improves instantly.
In short, you become slightly superhuman as long as your brain perceives some type of stress or danger that might possibly affect you.
This happens automatically as an unconscious survival tactic. However, humans do not live in nearly as chaotic an environment as we did when we lived in caves