It's no longer available on Play Store and Samsung has moved on and created new app named Samsung Flow. Though SideSync is still available to download for PC.
SideSync is a new PC-Mobile solution that enables screens, windows, and data to be shared easily. Share between a PC and a Galaxy smartphone, or between a Galaxy Tab and Galaxy smartphone.
Using SideSync, you can conveniently share the screen and data between your PC and mobile device. Receive alarms of your phone through PC and use various features of your phone on the computer. Enjoy the smarter and more convenient life with SideSync.