API Application Programming Interface. Simply an api is simple interaction between a server and application or browser. A fine example is sending a tweet, The browser and IOS or android app actually use the twitter api. A POST response would be used that sends and authenticates the user and tweet text. Posting the tweet, which with the use of Twitters api can be done with just some code and no GUI.
A GET request using an api would see data pulled, its quick and uses little to no resources. These requests too need to be authenticated to make sure that the requests don’t get spammed say 1 request per second type of restriction.
Using the YouTube api would be how the ProTube app is made, you get most of the functionality of YouTube but get to skip the ads.