Stress is marked:
When you mark your last Stress box, you clear all your Stress and take a Trauma and a Completion
You can only reduce stress by making a Relieve Downtime Move when in a safe place to do so.
If you mark more than 3 stress in a scene (combat or otherwise), you are exhausted and may not use any further stress until you have rested:
Exhaustion causes all your rolls to be rolled at Disadvantage. Exhaustion over-rules any other move that would change the kind of roll you would make except pushing