An API is an Application Programming Interface. APIs are used to allow systems to retrieve data from other systems.
It is the method by which you contact the other system and retrieve data from it. Nearly all mobile apps will use APIs to retrieve data which is shown within the app.
Single-page web applications will also use a lot of APIs to manage the data that is displayed within them.
These days, APIs are generally developed in two main formats: REST or GraphQL. These are two different methods for doing the same thing which is contacting a system and retrieving data from it or pushing data into that system.