You were watching a lengthy YouTube video and you had to leave your computer for some reason or had to disconnect before you finish viewing the video.  The next time you log in, you can start viewing the video from just where you had left it by adding #t=AAmBBs (where AA is the minutes and BB is the seconds). 

Suppose you have watched a video for five minutes and two seconds, it would be #t=5m2s.  Remember not to place any space anywhere.