The RF-Nano is an Arduino Nano with built-in wireless connectivity. It's the brain of all Candle devices.
If you buy one of these, make sure it has a real ATmega328P chip, and not a "compatible" one.
You can find newer versions with antenna connectors, which are great to getting increased range if you connect a larger antenna. It's a nice idea to use one of these with an antenna for the Candle receiver. More on that below.
Need more range?
You can find newer versions of the RF nano that have an antenna connector. You can increase the range if you connect a larger antenna. Whichever antenna you connect, make sure it works for the 2.4Ghz frequency, and has a U.FL connector.
If you buy one of these, make sure it has a real ATmega328P chip, and not a "compatible" one. The fake ones are a little cheaper, but won't work.
Finally, with these newer chips you no longer have to select "RF Nano" when you use the Candle Manager. The reason for that checkbox is that the original RF Nano devices had two switched pins. Later RF nanos seem to have fixed that issue.