It's an auxillary power amp that normally would get it's signal from the preamp section of an integrated amplifier. For example, if you already have an integrated amp head (like a concert bass/lead, etc.) that you use to run a speaker cabinet(s), you can drive the slave amp with that head (provided it has some sort of preamp line out) and use it to power additional speaker cabs. You can't just use the slave amp by itself, it needs a preamp signal to work properly.
Clear as mud?