To summarize what is a retainer agreement, it can be structured in several different ways: Client pays a set amount each month to access a certain amount of time. Client pays a set amount each month for a specific set of deliverables. Client pays simply to have access to the freelancer.
A retainer agreement is a work-for-hire contract. It falls between a one-off contract and permanent employment, which may be full-time or part-time. Its distinguishing feature is that the client or customer pays in advance for professional work to be specified later.