Keyframes are the starting and ending points of your animations. They are represented by small, light grey bars along the dark grey bar of an animation.
They always come in pairs, known as a keyframe pair. A keyframe by itself doesn’t mean anything - it’s a static state. But two different keyframes show the change that takes place during the animation. Sometimes an animation will have more than two keyframes if the animation effect has multiple steps.
You can move keyframes around by clicking and dragging them.