Explanation

Report-only mode is a new Conditional Access policy state that allows administrators to evaluate the impact of Conditional Access policies before enabling them in their environment. With the release of report-only mode:
Conditional Access policies can be enabled in report-only mode.
During sign-in, policies in report-only mode are evaluated but not enforced.
Results are logged in the Conditional Access and Report-only tabs of the Sign-in log details.
Customers with an Azure Monitor subscription can monitor the impact of their Conditional Access policies using the Conditional Access insights workbook.
Reference:
https://docs.microsoft.com/en-us/azure/active-directory/conditional-access/concept-conditional-access-report-onl