You saved a video.
For this one you will have to do a little bit of work because the menus change, and not all provide you the ability to save. Here is a little step-by-step:
- Go to a video (any will do).
- Press to make the video go full screen.
- Press to bring up the "Up Next" bar, but more importantly, possible videos to save for later.
- Scroll down to the bottom row of up next videos, and begin pressing until you find one with the menu that has "Save for Later" at the top.
- Press that and unlock the achievement.