What is IK (inverse kinematics) in 3D programs

IK (Inverse Kinematics) is the movement and rotation of a chain depending to the position of the chain's effector. When working with IK, moving a bone or the effector recalculates the upper joint angles in the chain. With IK, you can position skeleton's foot with moving the foot to the right place like moving your leg to chosen sidewalk tile on the street.