By putting a lid on the pan the steam will raise the pressure within the pan.
Raise the pressure and the boiling point is also raised, which cooks your food at a higher than normal temperature. The time taken to cook food is reduced by up to 40%.
It’s the opposite to climbers on a mountain trying to cook, using standard camping gear, as the pressure is lower the higher you go, the food never reaches a high enough temp to cook properly.
The lid has a valve, calibrated to release pressure above a certain limit, to avoid the obvious danger of explosion. Don’t underestimate this! Steam engines were used for almost a century to drive our industries.