Despite the fact most of the form elements have a label built-in, controlled by an attribute, it still sometimes makes sense to want your label. One example could be having an interactive visualization on a form that requires user input. Wave supports standard, required and disabled states.
Check the full API at ui.label.
q.page['form'] = ui.form_card(
box='1 1 2 2',
ui.label(label='Required Label', required=True),
ui.label(label='Disabled Label', disabled=True),