Isobars are lines/areas of equal pressure represented on a weather map.

When isobars become very tightly grouped together it indicates a "tight pressure gradient" (steep slope).

The tightly packed isobars are due to the difference in air pressure between between High and Low pressure systems.

This "slope" or gradient indicates faster winds as air from the upper atmosphere mixes down to the surface of the earth.

Conversely, when isobars are very loosely grouped together the winds are typically calm.