An access map is a map of all the potential orientations a tool might take around a 3D point. When you want to place the tool tip at a 3D position, you will need to know how to orient the tool to avoid colliding with material you do not want to cut.
Each orientation in the map is similar to a latitude and longitude on a globe. The map is an image with each pixel representing an orientation of the tool at this latitude and longitude.
When the map is white, the tool with this orientation has access and will not collide with material it should not collide with. When the map is black, the tool cannot assume this orientation without colliding.