This probably refers to the atmospheric pressure at a specific location on Earth's surface.
This is basically read as a contour map. The contour lines are known as isobars, where the regions of high and low pressure meet, and is the path along which the air flows.
When the contours get more closely-spaced, that indicates a steep increase towards a relatively high atmospheric pressure as one moves towards the center region of the contour lines.
Regions with farther-spaced contours have relatively low surface pressure.
Further info is well-explained here: