Water in a sealed pressure cooker boils past 100°C — the ceiling you learned in school turns out to be movable.
Trapped steam does it. With no way out, it builds up and squeezes the water, and the boiling point climbs: one extra atmosphere of pressure is worth roughly 20°C.
Most cookers reach 15 psi, putting the water at around 121°C. Dinner cooks faster because it sits in liquid hotter than boiling water should ever get.