
Explanation

Supervision policies in Office 365 allow you to capture employee communications for examination by designated reviewers. You can define specific policies that capture internal and external email, Microsoft Teams, or 3rd-party communications in your organization.
You create supervision policies in the Compliance center. These policies define which communications and users are subject to review in your organization and specify who should perform reviews.
If you want to reduce the amount of content to review, you can specify a percentage of all the communications governed by a supervision policy. A real-time, random sample of content is selected from the total percentage of content that matches chosen policy conditions.
Reference:
https://docs.microsoft.com/en-us/office365/securitycompliance/supervision-policies