The Watch Later feature of Youtube allows users to rapidly add videos to a playlist and view them at a later point in time. Videos added to the Watch later playlist can be accessed by clicking on the Youtube menu > Watch Later. Once you have viewed a video you can either delete it or transfer it to another playlist.
To add a video to the Watch Later playlist:
Click on the small clock icon located at the bottom right corner of the video screen.
A small message will be displayed: "Added to Watch Later".