Animation requires showing the changing states of an object. Key frame in animation refers to the beginning and end point for the transition of an object. Simply put, the beginning key frame is where the object is right now and the ending key frame is where you want the object to be after transition. Animation that needs to be done within a decided upon time frame uses key frames to ensure that the transition stays within this timeline.
Why are key frames important? Simply because key frames contain the necessary information about where a transition should begin and where it should end on a timeline. A sequence of key frames is what viewers see as motion. An animator will start with establishing two or three key frames (or more, depending on the object and transition). Once that is fixed, the animator will then add intermediate frames to populate the transition and create a continuous illusion of the object’s motion