The "Bloom" effect (found in the "Quartz Composer") adds a dreamy quality to your video clips by creating a bloom of light in the brightest parts of the image.
iMovie Bloom Video Effect - Bloom
Effect In, Effect Out:To apply the effect to only part of the video clip, move the sliders to the places/times when you want the effect to begin and end. Leave them both set to 0:00 to apply the effect to the entire clip.
Radius: Adjusting the radius increases or decreases the size of the video frame. Moving the slider toward Max decreases the size of the frame; moving it toward Min increases the frame.
Intensity: This slider adjusts the intensity of the bloom effect. For a more intense bloom, move the slider toward Max; move it toward Min to decrease the intensity.