A “retainer” is a fee you’re paid to secure your services when they’re required, and there are many different kinds of retainer services.
Some freelancers operate subscriptions. You might sign up clients for a subscription service which guarantees them that you’ll create and post ten blog articles for them each month, for example. Or your clients could subscribe to a website updating service: you enter the company’s specials each month, or each day.
In essence, when he pays you a monthly retainer fee, a client knows that no matter how busy you are, the $500 a month (or whatever retainer fee you charge) ensures that he’ll jump to the top of your waiting list of clients. You’ll complete his project ahead of other projects you’ve taken on.