Sync stands for synchronisation. It is about getting two devices and/or machines to share data in an orderly way.
The idea behind sync is to get different assets (mobile devices, etc) to work and operate a system in unison.
Synchronising mobiles devices with a computer, for example, is usually done use a USB connection or a wireless communication technology, such as Bluetooth.
This will for instance, upload all you mobile contacts to your desktop and vice versa.