It means that the person has to make a (financial) contribution to the joint budget.
What does this phrase mean is the correct question. This is question in the present simple . The help verb does is followed by the subject which is followed by the base verb (infinitive).
Look at these examples:
To support mean[color=blue]s to assist. (postive statement in the present simple).
What doe[color=blue]s to support mean? (question in the present simple).[YSaerTTEW443543]