Why does YouTube Integration Essentials need to access the YouTube API? What is an API anyway?

An API is a way to have two pieces of software talk to each other to
exchange information. In this case the add-on contacts YouTube to ask it for
the thumbnail, the title and the description of the video added. The thumbnail
and title are visible on the webpage (in the player) while the description is
in the HTML as microdata for search engines optimization reasons.