To try to make this more clear.
Let's take a YouTube example of the url
https://www.youtube.com/watch?v=axhzZyNII_I
In order to embed the above YouTube video within a post on this site, you would use the syntax of
[youtube]axhzZyNII_I[/youtube]
Which would then allow the video to be embedded directly within this post.
You only need to include the unique code from the YouTube URL between the tags and not the entire URL itself. Which in this case is the code "
axhzZyNII_I"