A REST API exposes things as URLs, such as /devices/42/telemetry, and uses HTTP methods for actions: GET to read, POST to create, PUT or PATCH to update, DELETE to remove, with JSON bodies and standard status codes. The style is stateless, so every request carries what the server needs, and it maps cleanly onto caching, load balancers and authentication schemes the web already has.
In IoT the REST API is usually the northbound side of a platform: dashboards, mobile apps and enterprise systems query device state, change configuration and trigger commands through it, while devices themselves talk MQTT or CoAP. The limitation is the request-response shape: for the server to notify an application of an event, a webhook or streaming channel is added alongside the API. The IoT platforms guide describes this split.
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