A retainer flips the usual order of freelance billing: instead of doing the work and then invoicing, the client pays first to secure a block of time or a scope of work for the period ahead. That prepayment is what the retainer invoice collects.
Retainers suit ongoing relationships — a monthly design allowance, a support arrangement, a marketing engagement — where the client wants guaranteed availability and the freelancer wants predictable, recurring income. Unused time is handled by the terms you agree (it may roll over, or expire at period end).
In KAZI, a retainer can be billed as a recurring invoice so it issues automatically each period, and it sits alongside the client's projects and history in one record.