When a clip is loaded a Timeline is shown at bottom of the Preview. It expands over the full area limited by the animation range. You can move the Playhead by dragging with LMB.
The Timeline is composed of the following visual elements:
Blue line: Playhead
Yellow: Motion track
Yellow line: Keyframe
Orange line: Shape keyframe
Purple: Prefetched frames
Light green line: Solve start/end keyframe