The AY-Sound is the same soundchip as in the Spectrum 128. So this interface is for Spectrum 48k or clones that have no soundchip. With this interface you can listen to all tunes for 128k machines or use sound programs like Soundtracker, ProTracker or Sample Tracker etc.
Of course this interface does not give you more memory, so you can play only programs that fits into 48k memory. Fortunately most sound programs and demos use only 48k.
Also usable for users of 128/+2/+2A or +3, as the output of the interface is ABC-Stereo or ACB-Stereo and so superior than the mono-output the computer normally offers.
The interface simply plugs into the Spectrum's expansion port and has a through connector, so that you can use it with the DivIDE to store the AY music files (for example). You then need to connect some standard PC speakers to its sound out socket to hear the sound and adjust volume!