GeoSeal is a feature that allows Checkins/Checkouts only if device is within a preset geographical area at the time of the Punch. For example, if you add a new Location with Radius as 300 (meters), then this creates a Geofence of 300 meters around the Location. If GeoSeal is enabled, only punches from within this Geofence will be allowed:
Now consider two Members, one named A and the other named B, have this Location mapped to them (either as Base or Allowed Location). Both A and B have GeoSeal enabled in their Security Settings. This Location also has GeoSeal enabled in its Location Settings. Now if A and B try to Punch Attendance from the Location, with A physically being within the Geofence and B being outside, A's Checkin/Checkout will be accepted. B's Punch will not be accepted and will be prompted to seek Approval of Supervisor for accepting the Punch.
You can watch a tutorial on Checking In and Out by clicking here.