In vSAN, witnesses are an integral component of every storage object when the object is configured to tolerate at least one failure and when using mirroring as the Failure Tolerance Method (FTM). Witnesses are components that contain no data, only metadata. Their purpose is to serve as tiebreakers when availability decisions are made to meet the Failures to tolerate (FTT) policy setting, and they’re used when determining if a quorum of components exist in the cluster.
In vSAN, storage components can be distributed in such a way that they can guarantee availability without relying on a witness. In this case, each component has a number of votes—at least one or more. Quorum is calculated based on the rule that requires "more than 50 percent of votes."