API : Applicaton Programming Interface
When you build a machine, you have to provide controls so the user can make use of all its features.
For example, a TV has a remote so the user can turn it on or off, pick a channel, and adjust the volume.
These things are called interfaces. You could say they're the channel of communication between machine and human.
Not only the user entering commands, but the TV's screen monitor for getting back results and information also counts as an interface.
From a software point of view, when you turn on a computer or smartphone, there are software devices like buttons, scrollbars, sliders, and browser windows so users can control programs, sites, and apps the way they want and see information.
And between machines, and between software and software, tons of requests and information exchanges happen too.
To exchange information, software also needs a channel of communication with other software.
For example, say there's a Korea Meteorological Administration server that manages weather data.
Portals and all kinds of other websites and apps that provide weather-related services request and receive weather data from this server in real time.
Through pre-written software, information gets requested and sent between this weather server and these servers or apps.
Then there has to be a specified format (date, region, what to look up, etc.) for requesting information from this weather server.
If there's a public manual saying you get information in this format, anyone can refer to it and build software that uses the weather data.
Like this, the means by which software can receive requests and commands from other software in a specified format is called an API.
It doesn't only exist over a network.
Browsers also take instructions for certain actions from JavaScript through Web APIs.
(※ I summarized the script of the video below.)