To wirelessly send signals from one place to another we use a device called a transmitter. Some transmissions contain some form of intelligence (e.g. Wi-Fi, Broadcast Radio, Walkie-Talkie, GPS) whereas other transmissions are already useful without any additional information (e.g. microwave oven, MRI scanner).
What is RF transmission ?
The term ‘RF’ (Radio Frequency) applies to all these forms of transmissions. RF is any signal, somewhere between 3kHz (most people can easily hear that as a high pitched tone) and 300GHz. RF can be artificially generated (man-made) but RF is also constantly generated by the world around us (lightning, radiation of the sun, cosmic background radiation, etc..).
The frequency (or band) where the transmission happens determines to a large extent how the signal behaves. In general, lower frequencies have a higher level of penetration than higher frequencies. It should be obvious that given the same frequency, more powerful transmissions will reach further than transmissions with less power.
That said, when using radios in the ISM band, most users don’t have much choice in frequency nor power levels. To stay legal, use a Moteino with a radio module operating frequency that matches the ISM band plan in the region where it will be used. The maximum RF power output for these radios is around 100mW (milliWatt) and that is usually also the maximum in the ISM band for these devices.