That depends on what type of map you are referring to.
On a weather map it's usually part of a warm front.
On a road map they could be any type of road the cartographer specifies.
On a topographical map they could be elevation lines.
On the Washington Transit Map it's the red line train.
Check the legend and it will more than likely tell you.
The black lines mean it is a state boundary.
air pressure
Hachured contour lines on a topographic map indicate a depression or a hole in the ground. They are lines that have small perpendicular lines extending from them, pointing inward toward the depression or lower elevation area they represent.
state border.
the Borders
lakes probably
A map with contour lines is called a topographic map. These lines represent changes in elevation and help to visualize the shape of the terrain. Topographic maps are commonly used for hiking, surveying, and land navigation.
The heaviest contour lines on a topographic map indicate the highest elevation on the map. These lines represent the peaks of mountains, hills, or other elevated land features.
Swirly lines going to the center where the eye is.
Contour lines that are closely spaced
on a map,what do black lines between states usually represent? state boundries
On a map that uses contour lines to represent height, a flat plain should show very few (if any) contour lines.