API Gateway
A single ingress for clients — routing, auth, rate limiting, response shaping — in front of N internal services.
If external clients talked directly to dozens of internal microservices, every client would need to know each service's location, handle authentication for each, and cope with the internal topology — and every service would have to re-implement cross-cutting concerns like auth and rate limiting. An API gateway solves this by being the single front door: one entry point that sits in front of all the services and handles the concerns that would otherwise be duplicated everywhere.
A hotel front desk: guests make one request at reception, which routes it to housekeeping, the kitchen, or concierge — they never wander the back corridors themselves.