A barometer measures air pressure in units of millibars (mb) or, sometimes, inches of mercury (inHg). The salty term for it is glass. It will have one dial that moves to show the pressure and another that is set on top of the first, to record its reading, and then does not move. A barometer reading will vary with altitude, since pressure declines as you rise. Sailors, however, are at sea level where the pressure is usually about 1,013 mb (or 29.92 inHG). A glass should be calibrated at the current level from a live weather report.
High air pressure generally means more stable, fair weather. Low pressure is associated with unsettled weather, clouds, and wind. Much more important than the air pressure is how that pressure is changing, and whether the change is slow (less than 3 mb in two or three hours) or fast.
A rising glass predicts improving weather. If a slow rise, clearing skies. If a fast rise, breezy. A steady glass means conditions—either good or bad—will stay much the same. A falling glass signals that the weather will get worse. If a slow drop, lowering clouds with rain are likely. If a fast drop, expect strong winds, squalls, or storms. A very fast drop of more than 5 mb in two or three hours might signal an approaching severe storm or gale.
Confirm the glass reading with a look at the sky. If the glass is falling, watch for lowering or thickening clouds that show a front is coming. Falling fast with towering clouds means take action now because a squall or gale is coming soon. If steady and low with overcast skies, expect lingering bad weather. If rising and the horizon is clearing of clouds, the weather will improve.