A fully managed (PaaS) stream processing engine that uses a SQL-based language with temporal functions. It reads from Event Hubs or IoT Hub and sends results to Blob Storage, Azure SQL Database, Cosmos DB, or Power BI. It also runs on IoT Edge.
In DP-900 it is one of the real-time analytics services, along with Event Hubs (ingestion) and Microsoft Fabric Real-Time Intelligence (Eventstream, Eventhouse with KQL, and Activator).
Azure Stream Analytics is a concept that can show up on the exam. Check the full guide to see it in context.