In a normal cooking environment when moisture (water) rises to 212 degrees it turns to steam and leaves the cooking pot or pan. In a sealed pressure cooker pot however the steam cannot escape and so it remains inside the pot causing the pressure to build. That steam can further increase in heat pushing well past the boiling temperature.
As you might expect when the temperature increases inside the pot foods tend to cook faster.
But that is not all, in addition to the temperature being raised the high pressure forces a lot of moisture directly into the food that is being cooked. This also help the food cook faster, tenderize, and moisten quite effectively.