This would show up on your post as a screen with the familiar arrow, and clicking it would play the video for you. Now, you insert the code and then give the link a name, and a link shows up on your site. That’s it. No graphic of any kind, plus the link takes the viewer away from your site.
In order to get the display of the video the way it was before, you have to copy the code labled “embed” (on the YouTube page) and paste it in the HTML editor. Then, all is well again. 😉