Clack is an informal term for the sound made when a key is struck. The sound comes from the impact of the switch's mechanism when it comes to a halt. Metal contact switches in particular are noted for producing a very sharp, loud sound when bottomed out; the switch is technically not noisy, but rather the damping effect from rubber dome switches allows them to operate with significant less noise. The sheet steel mounting plate in many keyboards amplifies the switch sound; omitting the plate alone cuts out a fair amount of sound, as the PCB flexes and absorbs the impact of keystrokes.
The sharp sound of many switches leads people to mistake them for being clicky; it's common for vintage linear and tactile keyboards to be advertised for sale as "clicky" simply because they are significantly louder than the average rubber dome keyboard. The concept that clicky switches contain a dedicated clicker mechanism is not widely known. The noise level of a linear or tactile keyboard can in some cases be louder than another model of clicky keyboard.
Clack sound can be reduced by various damping techniques.