Read's meeting reports are designed to be shared with others, and the default/recommended setting is to automatically share reports with everyone that was invited to the meeting. However there are many reasons why someone may want to keep a report private to themselves, which is where the Make report private option comes in:
You can find this setting from your Calendar page, by clicking on the Access button for an upcoming meeting that you're the owner of:
If the meeting is part of a recurring series, you'll also be prompted after saving your changes if you want to apply those changes to all future meetings in the series or just that instance.
How it works
If you enable the "Make report private" setting, it will disable all participant access by setting both internal and external participants to "None". It will also prevent anyone else from adding Read to the meeting, meaning only your sharing settings can be applied. You may still choose to share the report with specific people or teams on the Share Report tab. Also note that this will not prevent team sharing if enabled via the Team Report Access Workspace setting.
Read looks at who created the video conferencing link in order to determine who the report owner should be. You can only use this option if your predicted role is listed as "Report owner". Anyone else who is not the owner will no longer be able to turn on the "Add Read?" toggle, and they will see that the meeting is marked as private with this chip:
Alternatives
If you didn't toggle the "Make report private" setting in advance of your meeting, you can always go into the report after the meeting and use the share button to revoke access from others. The report owner is the only one who cannot have their access revoked.
You can also adjust your sharing preferences in your account settings or for specific meetings on the Calendar page to not automatically grant access to internal or external participants. However keep in mind that this setting will only apply to reports that you own, and any other user who is invited to the meeting can still add Read themselves, which automatically gives them editor access. The only guaranteed way to prevent others from getting access to a report is the "Make report private" setting.