How can I use (Teensy 3.1) to communicate with Arduino?

PaulStoffregen in this thread suggested using RX/TX but what does it mean to "use" RX/TX?

Are those pins interrupt driven on the Arduino? Do I need to follow some protocol?

RX/TX are the serial/communication lines (pins)

I think pins 0 & 1 (although that could depend on the Arduino you are using.. but sure what pins they are on the Teensy. (just got my first Teensy 3.2 a few days ago.. and havent touched it yet)

Protocol? Yes. Start here: Serial Input Basics.