Plain English Meaning
It is how the altimeter turns changes in air pressure into a reading in feet. As you climb, the air gets thinner, a small sealed capsule inside the instrument expands, and that movement drives the needle to show a higher altitude.
Full Definition
The way a pressure altimeter works: a sealed aneroid wafer inside the instrument expands as outside air pressure decreases with altitude and contracts as pressure increases. This movement is mechanically linked to pointers on the face of the instrument, which display altitude in feet. The pilot sets a local sea-level pressure value in the Kollsman window so the displayed altitude is referenced to that pressure setting.