It's exactly as it suggests. But we'll simplify it for ya'. Say you need to plug your soundcard outputs into an external mixer or what not to monitor the sound in your computer, we need to set the soundcards operating level to match your monitoring device.
Most audio gear operates at either (-10 or +4 (+4 being the better standard), and some gear is switchable too). You need to consult with the documentation of your external gear to determine their I/O level (although it is usaually fairlly obvious as the levels will be either way "Hot" or way "Quiet" if set wrong. Note: these settings are not just for monitoring obviously, but for any audio patching operation (i.e. FX routing, etc.).
The consumer setting is designed for use with such gear as Home Stereos (You know, the cheaper ones that you find at the local Wally-World), whose operating level is usually somewhere between the 2 standards. More expensive component style systems are usually referred to as "Prosumer", as they often incorporate, professional standards (i.e. operating level selection, better EQ, etc.), with consumer specific functions (i.e FX algorithms, pretty lights, etc.)
