This is simply a very effective and functional SDR upconverter. Since, you’ve reached this post, I presume that you are aware of the purpose of any upconverter: You simply take one RF band and you shift it up in frequency without changing any of the signal characteristics that it carries.
Operation in brief: This upconverter collects signals from an antenna, it filters them with a 55MHz LP filter, it amplifies them (selectable) and then shifts them (i.e. through mixer upconversion action) into 200MHz to 255MHz band. You then send this RF band at the input of a SDR USB-dongle and there you are listening to the entire HF spectrum at your computer!
You need an HF upconverter because most SDR dongles can’t go lower than about 28MHz or so. If you want to listen to lower frequencies on the SDR dongle you simply can’t, the device isn’t able to hear anything lower than (say) 28 MHz. So, the solution to this limitation is to use an upconverter: It takes the HF band and it shifts it up within a frequency band that the SDR dongle can listen. Connect the SDR upconverter between your HF antenna and the SDR USB dongle. These are all obvious things, but newcomers into ham radio can easily get lost if they don’t understand the fundamental issues and requirements of an RF upconvertion function.