A hybrid SDR is a radio system utilizes a mix of conventional analog components as well as software and digital technologies to process a radio signal. The picture below shows a block diagram of such a radio.
The RF signal is first amplified and then frequency translated to a range that can be processed using the analog-to-digital converter. An amplifier might precede the ADC to provide some gain.
An FPGA subsequently processes the digital output from the ADC, conditions it for transport across a serial or parallel interface, and passes it on to a host-processor for further general purpose processing.
Most SDRs on the market today such as HackRF for instance are hybrid devices.