I will try to explain...
If you are typing a post and highlight something in
bold you will see either side of the word you want to be bold a pair of square brackets with B and /B in them. These are to indicate where the bold type should start and finish.
Similarly with the youtube tags - you put yt and /yt without spaces in square brackets either side of the youtube info, eg
using the bit after the ?v= in the youtube address such as OGt_6foIZDU in this case and it comes up with the link when you post.
Enjoy!