iMovie for Mac: playhead
The white vertical line that marks the current position in the browser or timeline. You can move the playhead by dragging it or clicking another area of the browser or timeline. You use the playhead to scrub, or move through a movie or clip and play it back from different points.
The playhead is not the same as the skimmer, which is the vertical line that appears as you move the pointer over clips in the browser or timeline.
The playhead is fixed in place unless you move it or click elsewhere.
You can skim to see what’s in other clips while maintaining the playhead position in the browser or timeline. While you’re skimming, the playhead dims. If both the playhead and skimmer are present in the same clip, the skimmer becomes the default position for playback or editing.