One front door for several services
An API gateway gives an app one place to send requests to a system made up of several services. Like a receptionist, it directs each request to the right place: an order request to the orders service, for example, and an account request to the accounts service.
The gateway can also check access, limit how many requests an app makes, and record requests to help diagnose problems. This keeps common rules in one place.
When the gateway has a problem
If the gateway fails or becomes overloaded, requests may not reach any of those services. Teams design it to handle heavy traffic and leave decisions about orders or accounts to the services responsible for them.