What is an API key and why do I need one all of a sudden for YouTube Integration Essentials?

Up before version 1.1.0 the v2 YouTube API was used, but in September 2014
Google announced it was going to decommission that version, forcing the switch
to v3. A big difference between v2 and v3 is that, whereas v2 allowed
"anonymous" requests, this is not the case for v3 any more. This means any
request to the YouTube API v3 has to be identified with a key (a string with
seamingly random letters & numbers). You can get such a key from Google for
free (see next question) and you have to enter that into this add-ons options
page in XF ACP.