Explanation on what vSyncTime() does

[prashantpalikhe]

Can we please add explanation on what exactly vSyncTime() does within

needsRendering: function() {
    return vSyncTime() && (needsDomRerender() || hasEventLoopEventsToDispatch());
}

Can we please add explanation on what exactly vSyncTime() does within

needsRendering: function() {
    return vSyncTime() && (needsDomRerender() || hasEventLoopEventsToDispatch());
}