Software Defined Radio (SDR) is the application of Digital Signal Processing (DSP) to radio waveforms. It is analogous to the software-based digital audio techniques that became popular a couple of decades ago. A single software radio platform can be used to implement virtually any wireless technology (Bluetooth, GSM, ZigBee, etc.) The device has the ability to transmit and receive over a wide range of frequencies. It’s like a very fast sound card with the speaker and microphone replaced by an antenna.
Source: https://thehackernews.com/2012/10/intercepting-traffic-of-widerange.html