In the video please introduce yourselves, explain what you're doing and why, and tell us anything else you want to about the founders or the project.
The video should be one minute long and should be uploaded to YouTube. If you don't want other people to find it, mark it as unlisted. Don't mark it as private or disallow embedding, or we won't be able to see it.
The video should contain nothing except the founders talking. No screenshots or postproduction wizardry please; we don't want this to turn into a video making contest. If you're going to spend time making something cool, put it into your demo/product.
Please do not recite a script written beforehand. Just talk spontaneously as you would to a friend. People delivering memorized speeches (or worse still, text read off the screen) usually come off as stupid. Unless you're a good enough actor to fake spontaneity, you lose more in the stilted delivery than you gain from a more polished message.
Please do not add any background music to your video. We regularly give up 10 seconds into a video because we can't stand the random background music.
Before submitting the video, you should try playing it yourself, and check that you can hear what you're saying. A significant fraction of the videos we get are inaudible. If you're going to apply at all, you may as well apply with a video we can hear.
Here are some successful YC application videos:
Campus Job (YC W15)
Flip (YC W15)
Zenefits (YC W13)
Teespring (YC W13)