This is a 30 MHz four section elliptic low-pass filter, combined with a two-section shelf filter, and followed by a medium-gain preamplifier. The low-pass filter serves as an anti-aliasing filter for a SDR, especially one that is clocked at 66 MHz or so. The shelf filters will provide moderate attenuation at the lower frequencies, where powerful signals can cause overload. The amplifier provides enough gain to compensate for the filter loss, and boost the signal presented to the receiver. Depending on the antenna system, some SDRs may require additional gain for optimal performance.
For more details, see the Filter-Preamp User’s Guide.