The Thunderbolt 3 (USB-C) to Thunderbolt 2 Adapter allows you to connect your Thunderbolt 1 and 2 peripherals for the new series of MacBook Pro's that support Thunderbolt 3 like the:
Apple 13" MacBook Pro No Touch Bar (Late 2016)
Apple 13" MacBook Pro With Touch Bar (Late 2016)
Apple 15" MacBook Pro (Late 2016)
Here's a look at what's inside the Thunderbolt 3 (USB-C) to Thunderbolt 2 Adapter's packaging and some close-ups of the device.