A gateway is a component that acts as an entry point between different networks or services; in web development it usually means an 'API gateway'.
It becomes the single front door for requests heading to multiple backends (microservices), handling cross-cutting concerns like authentication, routing, rate limiting, and logging in one place.
An API gateway can be seen as a specialized reverse proxy. Where a reverse proxy focuses on forwarding, caching, and load balancing, an API gateway focuses on application-level functions — validating access tokens/JWTs, transforming requests and responses, protocol translation (REST↔gRPC), plan-based rate limiting, API-key management, and request aggregation.
In a microservices architecture, a gateway means clients need to know only one stable entry point instead of the location and auth scheme of dozens of services. Handling authentication/authorization at the gateway avoids duplicating security logic in every service. Because the gateway can become a single point of failure and a bottleneck, it's usually run as multiple instances behind a load balancer for redundancy. (In general networking, a 'default gateway' means the router out of a subnet — a different sense of the word.)