Many people are surprised to learn that bone has a blood supply. It’s every bit a living tissue as skin, your brain, or your heart. And like any of those other tissues with a blood supply, a bone can be bruised.
Lots of people are familiar with a bruise on the skin. If you bump the outside of the skin hard enough it will cause tiny blood vessels in the skin to break, leading to some bleeding in the skin. This causes the common black/blue/green discoloration. If you hit a bit harder the force can be transmitted deeper, to the underlying muscle. And if you hit harder still that force can go straight to the bone.
A bone bruise is an injury to the interior of the bone (the bone marrow) with enough force to disrupt the internal blood supply but not enough force to crack the outside of the bone.