A retainer, or upfront deposit, is money that belongs to the client, but which the client has given to you to hold or to use to purchase services or products for their cases. In other words, it is money you have received, but not yet earned.
A retainer is a liability on your books even though you deposit the funds into your checking account. As such, RB posts it as a liability to the Client Retainers account. You manage retainers through the Collections pane of the Firm window.