You can learn more about API Gateway from the following links to the developer guide:

• https://docs.aws.amazon.com/apigateway/latest/developerguide/api-gateway-set-up-simple-proxy.html
• https://docs.aws.amazon.com/apigateway/latest/developerguide/set-up-lambda-proxy-integrations.html
• https://docs.aws.amazon.com/apigateway/latest/developerguide/api-gateway-create-api-as-simple-proxy-for-lambda.html

You may read more about REST principles and also understand the value provided by API Gateway by looking into how you would create them without API Gateway at https://javajee.com/book/introduction-to-restful-web-services-and-jax-rs.