Know-How Exchange : Copywriting

I am a financial copywriter based in London and one of my regular clients has offered me a retainer to keep open a few days a week to work exclusively for them. I'm not sure how retainers actually work though - Do I just charge my daily rate for the two days they want me to set aside, which gets paid even if some weeks they don't have work to offer me? I'd appreciate any information on this topic.