It is really straight-forward - open a video in full-screen mode, double tap on the left side of the screen and you go back 10 seconds and identically doing it on the right side takes you forward, just like the aforementioned "J" and "L". The feature is part of the YouTube app and it appears Google decided to mostly slip it though the cracks, as there is no official announcement yet.
It appears it is a staged roll-out. You just have to get the latest version of the app available for your device and region and see if it is included. If you are on iOS, updating should also bring along the convenient new lockscreen controls for casting playback. Some problems with the skip feature have already been reported. Many users claim that it slows down the interface and the full-screen toggle. Hopefully, these kicks get ironed out, as it is a nice little feature to have.