If you want for example to play a video from minute 01 and 30 seconds just add the following line to your URL (your http link):
You will notice the cursor (marker) for the video will be positioned at 1 minute and 30 seconds in the video and it will start from there.
This is useful if you want to give a youtube video to somebody else to see it and you want to see it from that potition and not from the start (useful when the video is long).
If you want to start a video embeded with an iframe, just add the following at the end of URL within your iframe:
Note that 150 is in second and is the result of 1*60 + 30 (just transform minutes and seconds into seconds).