API, no sure if I exactly understand it

API to my understanding is a file that has the ability to be added on to. Things like Youtube and Twitter, etc But still my understanding is very limited so I did some reading up on this. Its the interfacing of files and sharing. Here's an example



Look, to be honest I still don't fully understand this concept, nor is my brain inclined like this. But it's pretty cool and it's wicked that we have technology like this.