iMovie (2013): Mark clips as favorite or rejected
If your clips contain sections that you don’t like or that you might never want to use in a movie or a trailer—because they’re blurry, for example—you can mark them as rejected. You can easily rate the clips you like and the clips you don’t like as you review them. When you’re ready to make a movie or trailer, you can focus on your best footage by choosing to display only those clips you marked as favorites.
Clips or ranges marked as favorites appear with a green line at the top, and clips or ranges marked as rejected appear with a red line at the top. Ranges of clips with an orange line at the bottom are in use in a project.

Mark clips or ranges as favorite or rejected
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In the browser, select a range, a clip, or multiple clips you want to rate.
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Do one of the following:
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If you like the selection: Choose Mark > Favorite (or press F).
In the browser, a green line appears at the top of frames you’ve marked as favorites.

You can also mark a clip during playback. If you press F when a clip is playing, the next four seconds of your clip is marked as favorite. You can also press and hold F; this marks a clip range as favorite until you release F.
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If you don’t like the selection: Choose Mark > Reject (or press the Delete key).
A red line appears at the top of frames you’ve marked as rejected.
Note: If you chose Hide Rejected from the pop-up menu to the left of the browser search field, clips marked as rejected disappear from view.
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Remove ratings from clips
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Make sure the clips you want to work with appear in the browser.
You might need to choose a new filter option, such as All Clips, from the pop-up menu to the left of the browser search field so that all the clips you need to access are visible.
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In the browser, select the clips whose ratings you want to remove, and choose Mark > Unrate (or press U).
The green or red line at the top of the clips disappears.