Welcome to the forums. For an overview of the RACHEL-Plus there’s a video here called RACHEL-Plus Overview where Jeremy has a covered most of the features available in RACHEL.
A basic overview is that the RACHEL-Plus is a device that provides a WIFI hotspot for users to connect to with devices like phones, tablets, and desktop computers. Once connected they can browse content installed to the RACHEL-Plus’ hard drive like Wikipedia, Khan Academy, and much more which is selected and installed by the administrator through a web interface. It has a battery which provides power when in areas that don’t have consistent power as well.
If you’re looking to use other hardware to run/install RACHEL it can get quite a bit more complicated than it is with the RACHEL-Plus. We do have the RACHEL images available for the Raspberry Pi at this post if you are looking at alternative hardware.
Hope this helps and please feel free to ask any questions.