Generally, the word "retainer" possesses two contexts and refers to both the contractual agreement entered into when hiring a legal professional as well as the monies that are provided upfront as an assurance of payment for services. These retainer funds are placed into a trust account with a lawyer or paralegal; however, it is important to keep in mind that the money held in trust, the 'retainer', is YOUR money.
As your matter proceeds, your lawyer or paralegal will incur disbursements (such as the costs of court filing of your documents, costs of travel, and many other expenses usual to the legal process). Additionally, your lawyer or paralegal will invoice you for fees as services are rendered in the course of your matter.
Every so often, perhaps monthly, but always after disbursements have been incurred or fees have been earned, your lawyer or paralegal will transfer money out of your trust account into the lawyer's or paralegal's general business account. The amount transferred shall never be more than the amount due on account.
The lawyer or paralegal is without legal right to transfer money out of the trust account unless the money is properly due as reimbursement for expenses paid out or fees earned for the progress in work performed whether as hours billed or a flat rate for performance of specific services as previously agreed between the lawyer or paralegal and the client.