– Margin sets the threshold of distance near the surface of the rigid bodies.
– Mesh, Cone, Convex Hull and Cylinder shapes have a visible gap around them.
– Capsule, Sphere and Box shapes embed the collision margins.

– In this example see how different values for Margin affect a simulation:

– In this example a one Blender Unit square is rendered behind the sphere (1 BU = 1 m):

Blender Rigid Body Sensitivity Margin