youtube thumbailer [youtube $video_key] tag , where does it go ?

hey everyone

im using the youtuibe thumbnailer on my blog https://fierth.com/a

I needed something that would create thumbails for youtube vids as a default to provide a graphic for my newsfeed icon

it states

First, how is the video embedded in the post? It uses the following syntax:
[youtube $video_key]

Second, how do we get the thumbnail itself after we extracted the video key? We build the image’s URL with the following syntax:
https://i2.ytimg.com/vi/$video_key/default.jpg

what does this mean for me , do i have to put that code [youtube $video_key] someplace in my index or style ?

Thanks for your help , you guys are great