"Microsoft Defender for Cloud Apps is a Cloud Access Security Broker (CASB) that operates on multiple clouds. It provides rich visibility, control over data travel, and sophisticated analytics to identify and combat cyber threats across all your cloud services." - Microsoft
In short, The Microsoft Defender for Cloud Apps portal is a place where you can integrate your Azure AD user accounts, devices, and other third-party cloud apps to see what your users are using and then potentially put a stop to it.
Learn more: Implement and manage Microsoft Defender for Cloud Apps