In wireless technology, this refers to the operation of a wireless transmitter without a license from the government (in the US, the FCC). While most radio frequency bands require a license to use, some bands are set aside for unlicensed use.
Because two radios in the same area, transmitting on the same frequency band, would normally interfere with each other, radios operating in the unlicensed bands must be designed to detect such interference and automatically cope with it in a way that allows both radios to share the band.
Technologies designed to be used this way include Wi-Fi and Bluetooth.
The most commonly-used unlicensed bands in the US are at 900 MHz, 2.4 GHz, and 5.8 GHz.