What is Your developer key in youtube-videos script?

Hi,
I try to use this https://www.sitepoint.com/displaying-youtube-videos-php/ article to use youtube video in
my laravel 5 app , but reading it I found next :

// .env  APP_DEBUG=true  app_name='Your app name (optional)' client_id='Your client id' client_secret='Your client secret' api_key='Your developer key'
Code (markup):

I suppose that client_id/client_secret parameters are this :
https://imgur.com/a/9S7p7Rn
Is it so?

But I ma not sure what is last parameter ?

 api_key='Your...
Code (markup):

What is Your developer key in youtube-videos script?