It is the number of days before a proposal's due date on which the automatic payment reminder is sent (the tooltip reads Number of days before the due date used to send email reminders). It is only a default: every proposal has its own Days Before Due Date for Email Reminders field (on the proposal's edit page), pre-filled with this setting's value when the proposal is created, and the daily reminder job uses the proposal's own value from then on. Changing the setting therefore affects new proposals, not existing ones.