SDR is a computer based radio scanner for receiving live radio signals with a cheap USB dongle with a RTL2832U chip. Depending on the particular model it could receive frequencies from 500 kHz up to 1.75 GHz.

More Info: Software Defined Radio wiki: https://en.wikipedia.org/wiki/Software-defined_radio

 Example of a SDR USB receiver, RTL-SDR BLOG V3.