There is a special area knows as the ‘Template Row’ at the top of each list table or checklist table (its margin is highlighted like: ). This is where you design or change how each row in the table looks.
Below the template row is a preview of the list table or checklist table. You can use the  icons on the left margin to add/remove rows (list tables), and the checkboxes in the left margin of checklists. This lets you test the table while designing it.