SDR - Software Defined Radio
In short: an SDR is a radio with no IF, Modulator, or Demodulator stages as we generally understand those terms: a receiving RF preamp feeds directly into a (very fast) A-to-D converter, which is connected to a computer DSP/CPU to tune a signal and extract the modulated audio or data; on the transmit side, the CPU and DSP generate the modulation directly, and feed it to an RF power amp. (Wikipedia)
Watch some videos about a SDR Project with the HackeRF One Hardware and understand better
the basics of an SDR Project.
Software Defined Radio with HackRF
A possible application could be the GNU Chirp Sounder.
Very interesting Link to an SDR Project:
Kurt Moraw, DJ0ABR (click menu SDR)
Project "Homebrew standalone
SDR-QRP-Transceiver" (m0nka; in german)
Setting up Raspberry Pi as a SDR Server with RTL-2832U USB dongle ...
read more on page of ZR6AIC.
RTL-SDR for Linux Quick-Start Guide:
read more and download ...
Linklist of different SDR Software Projects (Win/Lin/Mac):
read more ...