Once the pair is built, Chatter 2.0 is a working set of wireless communicators — no Wi-Fi, no SIM cards, no network charges.
Send text messages between units, encrypted, over long-range LoRa radio. Expect a range of up to about 2 km (1.2 miles) with a clear line of sight between the two devices; in built-up areas, walls and buildings typically reduce that to a few hundred metres.
Message more than two units. Every Chatter has a built-in contact book, so if friends have their own units you can all add each other — you are not limited to the pair from one box, though each conversation is between two units rather than a shared group chat. See Can I connect more than two Chatter devices together?.
Code it. Chatter works out of the box, but you can program it in CircuitBlocks to customise what it does — see What programming languages can I use with CircuitMess kits? for the routes from beginner blocks to advanced code.
Range questions are covered in more depth in What is the maximum signal range of Chatter 2.0?, and if the units will not find each other, start with My Chatter 2.0 isn't connecting to the other unit. What should I check?.