A vacuum breaker is a device that prevents water from being siphoned backward in a direction it is not desired to go.
This prevents contamination should the public drinking water system's pressure drop.
[3] A vacuum breaker typically contains a plastic disc that is pressed forward by water supply pressure and covers small vent holes.
Should the supply pressure drop, the disc springs back opening the vent holes (which let in outside air) and preventing backflow of water.
Spill-resistant vacuum breakers utilize a check valve and air vent inlet that allows air to enter before the internal check valve opens, thereby preventing water spillage associated with pressure type vacuum breakers.