When you arrange to pay a retainer for a lawyer, you are actually C. PAYING THE LAWYER IN ADVANCE FOR WORK.
To have a lawyer in retainer means that you, the client, are paying the lawyer a small amount in a regular basis whether, you need his legal services at that time or not. The lawyer, in exchange, will lend his legal services whenever you need them.
Putting a lawyer in retainer is the usual practice of small businesses which needs constant legal work but is not financially capable of hiring a full time lawyer.