GCP's fully managed relational database service. Supports MySQL, PostgreSQL, and SQL Server. Manages backups, patching, and automatic failover.
In CDL it's the equivalent of RDS (AWS) or Azure SQL Database. An example of a relational database PaaS on GCP. Difference from BigQuery: Cloud SQL is for OLTP transactions; BigQuery is for OLAP analysis.
Cloud SQL is a concept that can show up on the exam. Check the full guide to see it in context.