You can trim the frames from the beginning and end of videos that you record. You can trim the original video or save the trim as a new clip.

While viewing a video, tap the screen to display the controls.
Drag either end of the frame viewer at the top of the video, then tap Trim.
Tap Trim Original or tap Save as New Clip.
IMPORTANT: Trimming the original video permanently deletes the frames you edit from the video.
New clips are saved in the Camera Roll album along with the original video.