Bone weight
A value per vertex and bone that decides how much that bone drags the vertex along when the skeleton moves — the weights across all bones sum to one.
A vertex halfway down the upper arm might be driven 70% by the upper-arm bone and 30% by the shoulder, so the skin bends smoothly instead of snapping. The technique is linear blend skinning (LBS), the real-time standard; graphics hardware often caps it at four bones per vertex because that fits in a single vector.
Weights are "painted" by the rigger or computed automatically from distance to the bone. Badly set weights produce the classic faults: an elbow that collapses, a knee that twists through itself. LBS also suffers the "candy wrapper" effect, where a twisted joint pinches down to zero volume.