API for Application Programming Interface is a fancy name in order to say that an application, a program is open for people who would like to play with some of its features. Said as this it does not take you that far, so we are going to take one example.
Let's imagine that you would like to create a program which notifies you everytime that it is raining in order for you to go outside and take out your clothes which were lying out there in order to dry.
In order to create such program you will to have an access to a database which is indicating the weather in your current location. So currently, your behaviour is the following one, you are going on a website listing the weather, you look at the weather and then you are taking a decision.
Everytime that you are accessing to this website, it is not without any consequences for the owner of this website, in fact, you are just interested in the weather at a current time and by requesting this web page information your browser is making probably about 30 HTTP requests (CSS files, images, analytics solutions sets on, text elements...) so it does not seem to you like this... but it is. So you can imagine what are the consequence when many people like you have the same needs.
As a result some people out there thought about opening their service to developers like you in order to just get the information you need. As a result, it is faster for you, it is faster for them and everyone is happy. This service is what we call an API, a service in order to make some requests in order to play with the data as you need and just for this specific need.
API are using what we call HTTP requests in order to work. This is what we are going to see here down below with an example.