Wireless Fidelity (WiFi) is a simple concept and has been in use for a very long time, at least to some degree.
Essentially, it is communication that utilizes radio frequency as it's vehicle for delivery. An electromagnetic playground discovered in the early 1800's and finally used productively in the late 1800's.
Of course the technology has been refined since the early days. Back then the purpose was information via Morse code. It was one way (simplex) communication, not duplex, and definitely not an automated process.
Many examples can be considered WiFi networking. Frequency Modulation (FM radio stations), Amplitude Modulation (AM radio), Citizen Band (CB radio), UHF, VHF, and more. These technologies have been used for nearly 100 years to bring information to people quickly and effectively.
It wasn't until 1991 that WiFi networking as we know it became a reality thank's to NCR/AT&T. It was developed for cashier systems and was capable of mind boggling speeds of 2 Mbps.
WiFi utilizes radio frequency to encapsulate packets of information for transmission across a distance. The distance is limited by the frequency used, the environment in which the transmission travels, the power of the transmitter, and the sensitivity of the receiver.
For consumer based use, the standard frequency is 2.4 GHz (gigahertz) and quickly following suite is 5 GHz. These are unlicensed radio bands, which means they are free to use by anyone up to a certain power threshold without government permission. 2.4 Ghz is a highly shared frequency, there are many devices that utilize this frequency. Things like cordless phones, microwaves, alarms, car remotes, and even devices that weren't intended to benefit from communication emit electromagnetic interference in this range.
There are many other bands in use, but require licensing from the FCC to even purchase the equipment capable. Cell phones for example use 700, 750, 800, 850, 1700, 1900, 2100, and 2500 MHz depending on the carrier and service. ISP's and other large companies will create point to point wireless links spanning sometimes 100 km or more using 11 Ghz, 18 Ghz, or 21 Ghz for back haul purposes.
The frequency variance has it's benefits and drawbacks. 900 MHz for example, is an unlicensed band, mostly used by wireless internet service providers (WISP's) to offer high speed internet in rural areas where there is a lot of terrain fluctuation. The benefit to this is greater coverage in places where line of sight (LOS) is a problem at the expense of bandwidth (speed). Where as 5 GHz is capable of greater speed potential, but requires perfect LOS to achieve this.