A logical switch has a dvPortGroup mapping in the vCenter, one per VDS. When the dvPortGroup backing logical switch is lost for any reason, the logical switch will stop functioning and the logical switch in the vSphere Web Client NSX UI will have its status as out of sync. The Resolve button from the out-of-sync link will recreate a new backing dvPortGroup so that the logical switch can continue to work.
In older NSX versions, recreating the missing dvPortGroup can only be performed through REST API.
