Background sync automatically syncs your contacts in the background, without having to press the Sync button.
This is available only with iOS 7.0 or higher.
When and how often does Background Sync occur?
Background sync occurs a maximum of once a day, according to Apple’s policy, where there is minimal usage of 3G and battery. The app does need to be open in the background for it to operate. Sometimes it gets triggered when connecting to a computer or charging device (USB cable), or moving into an area with connected Wi-Fi. At this time, Apple does not give any schedule or timing to the programmers or users. We do know that if you open your app less than once a week or so, it gets triggered less.