The One-on-One meeting option lets HR/Admin to keep a track of whether the One-on-One discussion has happened between the employees and the manager which a standard function in a review process. By enabling this you can actually let the managers to log the date when the discussion happened and furthermore they can also write their notes from the discussion. 

Once recorded, HR/Admin can see a date on the On-on-One Meeting column in the Org dashboard which indicates that the discussion has happened on that date. HR/Admin can also notify the managers to complete and log their One-on-One discussions if that is still pending.

HR View

Manager View

Employee View