The option allows you to specify custom sorting order.
Default order (when no --order is provided) corresponds to -o "active:name:path"

Examples:
nmcli con show -o name
nmcli con show -o +name
  - sort connections by name alphabetically
nmcli con show -o -name
  - sort connections by name alphabetically in reverse order
mmcli con show -o active:name
  - sort connections first by active status, then by name
mmcli con show -o -path
  - sort connections by D-Bus path in reverse order