Rewind and fast forward YouTube video
On the desktop Left arrow & Right arrow keys retard/advance the position of the "playhead" in the timeline by five seconds and JKL keys by ten seconds (K is play/pause). You can also press the numbers to jump around in the timeline (0 is the beginning, 2 is ~20%, 8 is ~80% etc.)
In the YouTube app., tapping the left or right side of the video will retard/advance by ten seconds:
If you are using a browser on your mobile device to view YouTube content, the screen tapping method is not an option, but "scrubbing" options are available. Press the timeline indicator, hold your finger down and drag downwards to select "Hi-speed", "Half-speed", "Quarter-speed" and "Fine" scrubbing, then swipe left or right.