Bones is an animation term referring to points of movement. Much like how your body has bones which are your frame, things that are animated need “bones” to be their “frame.” The places where bones meet are “joints” which like in real life is where movement occurs. However unlike real life everything that you want to move in an animation needs “bones” and “joints,” including hair. However real hair has a preposterous amount of movement, which is very hard to duplicate digitally without having a lot of “bones” and using up a lot processing recourses. That’s why hair in most video games tends to be so static because the developers would rather put that processing power into loading half the level instead of rendering realistic hair.

TL;DR “bones” refers to points of movement in an animation.