This site is recognizing that the above is a url and adding the tags. The y-tube video id is the numbers and letters following "watch?v=" in the URL(web address). It's shown in dark red, above. Copy that from your browser's address box and from the "advanced reply", you'll see the youtube button. Click it and it brings up the youtube tags. Place your video id in between. Preview post and it should appear embedded. Don't include the stuff after the id that starts with "&"(usually followed by "feature" or "related", etc.).
Grammar: The difference between knowing your shit and knowing you're shit.