HD maps (high definition maps) are essential for self-driving cars. They have a high accuracy of object locations, up to 10 cm.
Self-driving cars use HD maps for multiple purposes. It helps to solve the localization problem: figuring out where exactly the car is in the world. GPS can have errors up to a few meters, which is an unacceptable accuracy for driving. With an HD map, the localization software can compare features of surroundings with the map and figure out where exactly it is located.
Then the map can provide valuable information on surrounding objects: where are traffic lanes, which lanes go straight, and which are used for turns only, what traffic signs are nearby, the speed limit on this road, the location of pedestrian crossings and bike lanes, height and location of the curb, etc. All this can be useful for path planning.