A Web API is an online “application programming interface” that allows developers to interact with external services. These are the commands that the developer of the service has determined will be used to access certain features of their program. It is referred to as an interface because a good API should have commands that make it intuitive to interact with.
An example of this might be if we want to get information about a user from their social media account. That social media platform would likely have a web API for developers to use in order to request that data. Other commonly used APIs handle things like advertising (AdMob), machine learning (ML Kit), and cloud storage.