An API, or Application Programming Interface, is an interface that allows applications to communicate with one another, with other programs, and with end-users. An API can be thought of as a contract governing how two programs communicate.
In a microservices architecture, each microservice has its own unique API. These APIs govern how all microservices communicate.
In traditional monolithic architectures, internal APIs were architecturally distinct from external APIs. External APIs were exposed using high level network protocols such as HTTP, while internal APIs were exposed using internal programming interfaces that were not accessible over the network.
In today's cloud-native architecture, there are many more network-accessible APIs.