KQL stands for Kusto Query Language. It's the language used to query the Azure Data Explorer, Azure Defenders, Azure log databases: Azure Monitor Logs, Azure Monitor Application Insights and others. Kusto databases are perfect for massive amounts of streamed data like application logs and telemetry database.
here is a short check list on how to start your journey with KQL, Kusto Query Language.
KQL basic and advance free on-line courses, start with the first ~20 operators that will be used 80% of your time and move forward to advanced capabilities with Scan, Geospatial, Time Series and others.