A keyframe (or key drawing in traditional animation) refers to the frame that shows the key movement of an object or a character. Often time is the frame that shows the extreme posture of a sequence of action. For example, when a bouncing ball reaches its highest point.
When drawing a complex movement, animators draw the keyframe first and fill in the inbetweens to make sure that the animation plays smoothly.
It’s not common to draw from the first frame to the last at once. You might find the characters in the last frame look very different from the first frame.