API (application programming interface) is a type of software interface offering a service to another pieces of software. Using this service we can do so many things. As a example we can create simple CRUD (create, read, update, delete) application, or we can output result of some process as a simple JSON or XML. Then the output can use in the other part of the software (Mobile app, Web app, Standalone app)
I will develop reliable and secure API for your application using PHP, Python or NodeJS.