The pressure gradient is the change in atmospheric pressure across horizontal distance, and the force it produces is the initial reason air moves from high pressure toward low pressure. The pressure gradient force is drawn perpendicular to isobars, pointing from higher pressure toward lower pressure.
The physics
Pressure at a point represents the weight of the atmosphere above it. When two nearby locations have different pressures, the atmosphere is out of balance and a force acts on the air to move it toward the lower value. The magnitude of that force is proportional to how quickly pressure changes across the distance between the two locations.
This gradient force does not act alone. Once air starts moving, the Coriolis force deflects it and friction near the ground slows it down. In balance, the wind flows nearly along the isobars in the free atmosphere, faster where isobars are packed close together and slower where they are spread apart.
How forecasters read it
On a surface analysis, isobar spacing is a direct visual estimate of wind speed. Tight packing on the map is often the first clue that a high wind event is possible, even without a thunderstorm. In a severe weather context, the low-level pressure gradient between a deep surface low and an anchored ridge to its east can drive a strong southerly low-level jet, which increases 0 to 1 km shear.
The gradient also shapes moisture transport. Return flow from the Gulf of Mexico into the southern Plains depends on how tight the gradient is between a low over the High Plains and high pressure over the Southeast.
Where it does not tell the whole story
Pressure gradient sets wind speed, but wind speed is only one ingredient in a severe weather forecast. A day with impressive gradient winds and dry air can produce fire weather and blowing dust rather than storms.
Very small gradients over the warm sector, in contrast, can still support significant severe weather if the shear profile is favorable aloft and instability is high. Read gradient alongside moisture and shear, not by itself.