It's an upconverter which means it takes frequencies that are low and then "converts" them upward (in a numerical sense, I suppose) - most RTL sticks and "cheap USB TV tuners" have a frequency range from 25 MHz to 1700 MHz, give or take a little more either way (the Elonics E4000 can reach to 2.4 GHz iirc).
A lot of people are interested in monitoring HF and SWL activity which runs anywhere from 100 kHz to 28 MHz - again give or take a bit - and since the "cheap USB TV tuners" can't tune that low with their cutoff at 25 MHz roughly you use an upconverter to take the frequencies that are naturally low and convert them to higher frequencies that
can be received by such hardware.
According to specs it shows a 125 MHz base conversion frequency so here's an example: you might be you wanting to listen to some shortwave station broadcasting at 7125 kHz (7.125 MHz) - the upconverter would convert that into a signal at 125 MHz
+ the frequency you want to monitor so you'd tune the RTL stick or whatever tuner you're using (being fed a signal from the HamItUp! of course) to 132.125 (125 + 7.125) and AM mode or sync AM depending and voila, you hear the HF signal like you were tuning it at the native frequency of 7.125 MHz.
Makes sense, right?