Which instrument measures atmospheric pressure?

A barometer measures the weight of the air above it. The original design, Torricelli's, was a glass tube sealed at the top and standing in a dish of mercury: atmospheric pressure pushing on the dish holds a column of mercury up in the tube, and the height of that column, around 760 millimetres at sea level, is the reading. Aneroid barometers, the common household kind, replace the mercury with a sealed metal capsule that flexes as pressure changes, moving a needle through a linkage. The reading matters for weather because pressure reflects what the air is doing vertically. Falling pressure means air is rising, cooling and condensing, which brings cloud, rain and wind; steady high pressure means air is sinking and drying out, giving clear settled conditions. Pressure also falls predictably with altitude, which is why the same mechanism, with a different dial, becomes an altimeter.

Weather maps convert every reading to its sea-level equivalent, otherwise a mountain station would always appear to be in a storm.