CAN = Controller Area Network
Basically a CAN bus allows bi-directional communication at about 1 Mbps between multiple devices. Until the 2015 season, the devices were limited to Jaguar speed controllers and you needed a bridge (2CAN or a Jaguar acting as a serial to CAN bridge) for the Jaguars to talk to the cRIO.
With the new control system, the RoboRIO has CAN built right in, and a number of the new control system components also use CAN. The new PDB has CAN-based current monitoring. The pneumatics module is controlled via CAN. New Talons also have CAN built in.
The implementation of CAN in the 2015-19 control system uses 2 wires (I believe they're Transmit + and Transmit -). One set of those wires connect from the RoboRIO to one of the CAN components. Another device connects to the first device and so on. You need a termination resistor at each end of the system; the RoboRIO has one, and the PDB has one as well, but it can be bypassed if necessary. (I don't see any reason not to have the PDB at the end of a system.)
As for programming, all the necessary libraries are in WPILib and are nothing like the CAN libraries from years past. I don't know much more than that, but hopefully someone on a beta test team can explain further.