I don't know where you would see this, but for instance a walk-in closet is one that you would be able to walk in to like any other room meaning it is big. Most banks you walk in to in order to do business unless you use thge drive up window. If you take walk-ins at a hair salon, you don't have to make an appointment so maybe that is what walk-in bank is like to start an account or to obtain a loan without an appointment.

The opposite of an online bank, i would say.