A pro-forma invoice is a non-binding document used to provide customers with a cost estimate before finalizing an order. It allows sellers to estimate pricing and terms upfront. In Odoo, pro-forma invoices can be created from regular customer invoices by clicking "PRO-FORMA" instead of "Validate." This changes the invoice status to pro-forma to distinguish estimated invoices from final ones. Pro-forma invoices help customers decide whether to place orders and allow sellers to lay out pricing variables.