What does "rf" mean in "rfkill"?
RF stands for radio frequency. As per the Linux Wireless Wiki:
rfkill is a small userspace tool to query the state of the rfkill switches, buttons and subsystem interfaces. Some devices come with a hard switch that lets you kill different types of RF radios: 802.11 / Bluetooth / NFC / UWB / WAN / WIMAX / FM. Some times these buttons may kill more than one RF type. The Linux kernel rfkill subsystem exposes these hardware buttons and lets userspace query its status and set its status through a /dev/rfkill. Given that at times some RF devices do not have hardware rfkill buttons rfkill the Linux kernel also exposes software rfkill capabilities that allows userspace to mimic a hardware rfkill event and turn on or off RF.
As a side note if it hasn't become obvious yet, Wifi and Bluetooth devices are in fact radios and use digital modulation such as QPSK/DPSK instead of AM or FM (source - I'm an EET student). Note also that rfkill is a userspace command to expose functionality available in the Linux kernel's RFKill subsystem:
Many computer systems contain radio transmitters, including Wi-Fi, Bluetooth, and 3G devices. These devices consume power, which is wasted when the device is not in use. RFKill is a subsystem in the Linux kernel that provides an interface through which radio transmitters in a computer system can be queried, activated, and deactivated. When transmitters are deactivated, they can be placed in a state where software can reactive them (a soft block) or where software cannot reactive them (a hard block).
RF stands for "Radio Frequency". The first transatlantic RF transmission was received in St. John's Canada in 1901. RF is used for radios, TVs (not so much these days), walkie talkies, cell phones, etc. From your point of view RF is limited in maximum range of 10 meters to 30 meters.
RF is often associated with Wi-Fi. Wi-Fi is simply a trademarked term meaning IEEE 802.11x. The false notion that the brand name "Wi-Fi" is short for "wireless fidelity" has spread to such an extent that even industry leaders have included the phrase wireless fidelity in a press release.
The misnomer of "Wireless Fidelity" by the masses is most likely caused by mental muscle memory of "Hi-Fi" which stood for "High Fidelity" in stereophonic equipment. (No source for this: just my theory).
Wi-Fi hotspots, Wi-Fi home networks and Wi-Fi business networks are used to connect computers and smartphones to the internet without wires. Therefore RF (Radio Frequency) becomes the backbone / transport layer that replaces the wires (Cat 5/5e, etc).
rfkill is simply a way of resetting computers and smartphones connect via Radio Frequencies (RF) to the internet. It can even reset devices such as speakers, mice, keyboards, etc. In this case the devices are connect with the "Bluetooth" standard.
Devices can also be connected to your computer with IR (Infrared) but this requires line of sight and doesn't use RF which bounce around. AM Radio bounces off the sky, FM Radio goes across the horizon. All types of RF are out there, including those from deep space. One discovered this year could be from an alien spaceship (if you believe that ;)).