A switch device might be a large button, a puff-and-sip tube, or even a sensor triggered by an eyebrow movement, depending on what movement a user can reliably control. Software scans through selectable options (links, buttons, menu items) one at a time, highlighting each in turn, and the user presses their switch at the right moment to make a selection.
Because scanning is slow by nature, switch users benefit enormously from a simple, predictable page structure, a logical tab order, and interfaces that avoid unnecessary steps, all of which overlap heavily with general keyboard accessibility best practices.