what is exactly API ?

An API or more aptly Application Programming Interface is basically a library of code that allows programmers to interact or get information from an external source.

Let's say you are tasked with creating a specific feature for your application. Say you have to get the weather in a logged in user's location, to determine if your app should give them coupons for say rainy day gear, or outdoor activity items.

Your app however, does not have a real-time database for gather weather. So after searching online you come across a database for weather in any location around the world from NOAA. It would be great if you could have access to their database, but that won't be possible since you don't work for them. However, after further research on their site, you notice that they have an API, perusing around on it you realize that you can send a GET request to their API and get back JSON of the weather in a location you specified in the GET request.

Using that you are able to send a GET request and retrieve the necessary data so that you can finish creating your feature.

What I just described is how you would use an API to get data from an external source that you don't actually have access to, but instead are given some access through the use of an API.