This expertise represents your character's ability to withstand and carry on in the face of great pain, injury, and physical hardship. It can also represent a resistance to torture, and general stamina and endurance. It is toughness and willpower, but it is not supernatural - you cannot achieve anything with Endure Pain that would defy physics or biology.
No Points: You can take a punch. You've probably been hit a few times and had to keep fighting in spite of it, though a debilitating injury would still be a massive hurdle. Harsh conditions like extreme temperature or averse weather affect you like they do most people of your species.
1 Point: You're made of tough stuff. Rather than simply being able to take a punch, you know how to roll with those punches and fight through the pain of a broken nose and bruised eye. You aren't going to ignore a major wound through sheer grit and willpower, but when you get knocked down, sometimes you get back up-- even when you shouldn't. Harsh conditions are a bit easier for you to deal with, and you're able to withstand a lack of basic needs for longer than average.
2 Points: You've been hit hard and kept fighting. A broken bone isn't enough to reliably take you out of the fight, and a grazing hit might as well be a tickle. Never count you out of a fight, because if all else fails, your raw determination to keep going through adversity is enough. At this point, harsh conditions are second nature, though you still have limits in extreme conditions.
3 Points: You're beyond stubborn-- you simply do not give up. No matter how hard you get hit, you just keep coming. You aren't just ignoring grazes at this point, you're being blasted at point blank and surviving to make that hospital trip. Pain is little more than a distraction. You've been through it all-- and you can do it again.
3☆ (Mastery): You would look a Sith Lord in the eye after being electrocuted, and with blood pouring from your lips and smoke rising from your skin, call them a wimp. This is the stuff action heroes of made of. Discomfort is nothing. Pain is motivation. There is no stopping you without knocking you out or killing you. No doubt your body is a litany of scars and it tells the story of the impossible odds you've survived, and somehow, you're still here, still fighting.