Looker is an enterprise business intelligence (BI) and embedded analytics platform that uses a governed, model-driven approach. Its central feature is a centralized semantic/modeling layer (LookML) that defines metrics once for reuse across dashboards, reports, and embedded analytics. Looker typically queries data in place (in a customer's warehouse or database) rather than copying data into the platform. Refer to Google Cloud documentation for implementation details.
Looker is maintained as part of the Google Cloud product lineup, which also includes Looker Studio (a separate, primarily self-serve reporting product with different pricing and target users).