The API gateway pattern is recommended if you want to design and build complex large microservices-based applications with multiple client applications. The pattern is similar to the facade pattern from object-oriented design, but it is part of a distributed system reverse proxy or gateway routing for using as a synchronous communication model.

We said that It is similar to the facade pattern of Object-Oriented Design, so it provides a single entry point to the APIs with encapsulating the underlying system architecture.

The pattern provides a reverse proxy to redirect or route requests to your internal microservices endpoints. An API gateway provides a single endpoint for the client applications, and it internally maps the requests to internal microservices.

In summary, the API gateway locate between the client apps and the internal microservices. It is working as a reverse proxy and routing requests from clients to backend services. It is also provide cross-cutting concerns like authentication, SSL termination, and cache.

https://medium.com/design-microservices-architecture-with-patterns/api-gateway-pattern-8ed0ddfce9df