In one sentence
An HL7 standard that defines healthcare resources and an API framework for exchanging them.
FHIR (pronounced 'fire') is a standard created by HL7 International for exchanging healthcare information electronically. It defines resources for clinical and administrative concepts and an API framework for interacting with them.
CMS and ONC require or certify FHIR-based APIs for specific programs and exchange use cases. That does not mean every healthcare workflow or EHR operation must use FHIR. Eligibility and benefit inquiries, for example, are still commonly handled through the HIPAA X12 270/271 transaction.
FHIR supports RESTful interactions and multiple representations, including JSON and XML. Each server publishes a CapabilityStatement describing the resources, search parameters, and interactions it supports. An endpoint may be read-only, expose only a subset of resources, or require customer and application approval before production access.
For automation work, FHIR is useful when the required resources and operations are actually supported. Scoping should verify the vendor's current capability statement, authentication, access agreement, write support, sandbox, and production approval process rather than assuming universal CRUD access.