A read-only query language optimized for exploring large volumes of logs, telemetry, and time series, with operators chained by pipes (|), such as where, summarize, and render.
In DP-900 it is used in Fabric Real-Time Intelligence Eventhouses and Azure Data Explorer. In SC-900 it is the language of Microsoft Sentinel analytics rules and hunting, and of advanced hunting in the Defender portal.
KQL (Kusto Query Language) is a concept that can show up on the exam. Check the full guide to see it in context.