Peer to peer technology has been around for years (remember Napster, anyone?). Simply put, the technology enables users to share data direct, computer to computer, across the internet. The files remain on your system and other users effectively download directly from your computer.
Resilio is the new technology and Sync is the application aimed at home users. There's a basic free version which will enable an individual to synchronise their data across their devices.
As an example, you could make sure your most important documents are in-sync across a PC, Mac and iOS device. There's even support for your NAS drive, if you want to quickly get files to/from the network drive or want to access them remotely.