Flask is a micro web framework for building web applications in Python. It is a lightweight framework that is easy to use and flexible. One of the use cases of Flask is to create RESTful API (Application Programming Interface) which allows communication between different systems.

A Flask API is a web application that is built using the Flask framework and follows the principles of a RESTful API. RESTful API is an architectural style that defines a set of rules and constraints to be followed while building web services. A Flask API allows a client (such as a web or mobile application) to interact with the server using HTTP requests (such as GET, POST, PUT, DELETE) to retrieve or manipulate data.

To build a Flask API, you can use the routing capabilities of the Flask framework to map URLs to Python functions that handle specific requests. These functions can then retrieve or manipulate data from a database or other sources and return the results in a format such as JSON.

Flask API can be easily integrated with other libraries and frameworks such as SQLAlchemy for database management, Marshmallow for serialization/deserialization and JWT for token-based authentication.

Overall, Flask API is a lightweight and simple way to create RESTful API in python, it is flexible and easy to learn and use, which makes it a popular choice among developers.