Rest API in Magento 2 defines a set of functions used by the developers to perform requests and get responses using the HTTP protocol.
Create custom rest API in Magento 2 gives some predefined rest APIs like Product API, Order API, Customer data API with which you can assume virtual responsibility for everything occurring on the site.
For more information visit: https://meetanshi.com/blog/create-custom-rest-api-in-magento-2/
