pressure
/ PRESH-er /
Press your palm flat against a wall and the same push spreads gently over a wide area; press a thumbtack with the same push and it bites in sharply. Pressure captures this difference: it is force spread over the area it acts on, force divided by area. A small force on a tiny area can mean enormous pressure.
For a gas, the pressure on the walls of its container comes from countless molecules hammering against the surface — each collision a tiny shove, but added up over a huge number per second they produce a steady, smooth push. Pressure is measured in pascals (newtons per square metre), and also in older units such as atmospheres, bars and millimetres of mercury.
Pressure matters because it is one of the handful of variables that pin down the state of a gas, alongside volume, temperature and amount. It is also why your ears pop on a plane and why a sealed can bulges when heated: change the molecular hammering, and the pressure follows.
Standing on snow you sink in, but spread your weight across snowshoes and you stay on top — the same body weight, but a far larger area, gives a far smaller pressure underfoot.
Same force, larger area, lower pressure — the idea behind snowshoes.
Beware the two meanings of pressure in chemistry: absolute pressure is measured against a vacuum, while gauge pressure (what a tyre gauge shows) is the amount above the surrounding atmosphere. Gas laws always use absolute pressure.