IK stands for “Inverse Kinematics”.
Inverse Kinematics differs from forward kinematics in the sense that you can grab and effector, or an object at the end of a bone/joint chain, and control the movement of the entire chain based on predifined rotation angles and constraints.
Picture an arm from the shoulder to the hand. With IK, all you’d have to do is move the hand, and the shoulder, elbow, and wrist would move appropriately.
With FK, you’d have to animate each joint postion individually (rotate shoulder, then elbow, then wrist, then hand).