REST API is an API (Application Programming Interface) that follows the REST (REpresentational State Transfer) architecture.
REST is known as REpresentational State Transfer because in a client-server communication model, any API following the REST architecture will create an object for the data requested by the client and also send the values of the object in response to the user, thus sending both the object and state of an object.
REST architecture is often called the "language of the internet" because it helps an application be more responsive as it leverages very little bandwidth and is completely based on resources.