An API publishes the operations a system supports, such as creating an invoice, looking up a customer, or listing documents, along with the format of requests and responses and the rules for authentication. Other software can then integrate with the system without access to its internals.
APIs make integrations, automation, and AI agents possible. Good APIs are documented, versioned so changes do not break existing integrations, protected by scoped credentials, and rate-limited to keep the system stable.