If you have a YouTube account, you can easily add videos you want to remember to watch to your Watch Later list in just a few taps. It's a nice feature for times when you come across a video you want to watch but don't have time to right at that second. Instead of weeding through searches in order to find it again, you'll be able to access it in just a few taps inside the YouTube app for iPhone and iPad.
Note that you'll need to be signed into a YouTube account in order to save videos to a Watch Later list. This ensures that they are synced across any device you may want to watch from later.
That's it. You'll now see a list of videos that you've saved to your Watch Later list. You can remove them after you're done by following the same steps you did for adding them in the first place.