Jon, Select one image and change the description. That image is the primary variant. Select all other images in the normal way. Cmd-A to select all or Shift-Click to select others, or Cmd-Click to select random images.
Still in the Metadata Tool select the Double headed arrow. That will bring up all the changeable fields within the Metadata Tool. Uncheck all the ones you do not want to change. (Only the fields that you have changed will be checked.) Then click apply and all the secondary variants will be changed.
If you have not made any changes other than the Description, in your example, you can hold down the Shift Key and click the Double Arrow and it will copy and apply your description to all the selected Variants. So, sometimes you can take advantage of the Shift Key option by ordering the changes you make in the different tools.
Also, make sure the Edit Selected Tool ("Toggle between Edit Primary and Edit Secondary Variants") is selected in the Tool Bar. The icon with the Multi-able Boxes.
Good Luck.