Atmospheric pressure is the weight of the air column above a surface. Weather maps usually express it in hectopascals (hPa), numerically equal to the older millibar unit.
A station's measured pressure depends strongly on elevation. Synoptic maps therefore use pressure reduced to mean sea level so that places at different heights can be compared.
Highs and lows
A low is an area with lower pressure than its surroundings. Air tends to converge and rise, often supporting cloud and rain. In a high, descending motion more often favours settled conditions.
This is not absolute. Fog and low cloud may persist beneath high pressure, while a dry low can produce wind without much rain.
Isobars and wind
Isobars join points of equal pressure. The closer they are, the stronger the horizontal pressure gradient is likely to be and the greater the potential for strong wind.
Actual wind direction is shaped by Earth's rotation, friction and terrain. It does not simply flow in a straight line from high to low pressure.
Trend matters more
A single value such as 1013 hPa does not describe the weather by itself. Position relative to pressure systems, the change over recent hours and the upper-air pattern are often more useful.
A rapid pressure fall may indicate an approaching or deepening low, but maps and observations are still needed.
These explanations and calculations are educational. For safety, occupational, health, marine or aviation decisions, follow the appropriate official services.