What are Kékulé numbers? I saw them in the menu and don't know what to do with them.

Kékulé numbers are explained in great detail on this website:
Kékulé numbers

In MacFamilyTree, you can use them to group close relatives in the persons overview list, for example.

  1. In the left-hand column, click Edit, then Persons
  2. The overview list will open
  3. Right-click (ctrl-click) the person you'd like to see the close relatives of
  4. Choose "Calculate Kekulé-Numbers" from the context menu
  5. Click the coggwheel button in the bottom right corner
  6. An Options window will open
  7. Make sure that "Show column for Kekulé Number" is activated
  8. Back in the overview list, click Kekulé in the head row
  9. That field will be highlighted blue and the overview list will be sorted by Kekulé numbers