Inverse Kinematics

Most character animation is created by rotating the angles of joints in a skeleton to predetermined values. The position of a child joint
changes according to the rotation of its parent. The end point of a chain of joints is determined by the angles and relative positions of the individual joints along the chain. This method of posing a skeleton is known as forward kinematics
.

However, it is often useful to pose joints from the opposite point of view. Start from a chosen position in space, or a goal, and work backwards to find a way to orient the joints so that the end point reaches the goal. This can be useful if you want a character to grab an object or stand on an uneven surface. This approach is known as inverse kinematics (IK). It is supported in Mecanim for a humanoid character with a correctly configured Avatar

copied from https://docs.unity3d.com/Manual/InverseKinematics.html
.