I'll try!
100V line systems are designed for distributed audio systems where you want to run lots of small, low volume speakers from one amplifier and often over long distances.
From what I understand, 100V is the nominal maximum voltage that the amplifier will drive. You could probably think of it like running a mains supply round a building, you can keep plugging in speakers until you reach the maximum power available rather than the amplifier can supply. The speakers used on 100V line systems will have a transformer between them and the line to step down from the 100V line to the required voltage (volume) for that speaker. If you look on the back of one, you'll usually see a switch that will say something like 1W, 2W, 4W etc. This is how much power the speaker will get assuming the 100V line is driving at full voltage.
Just like for mains distribution where the voltage is stepped up to 11kV and higher where long distances are to be covered, it's done for speaker systems to reduce cable losses. If you were just using conventional amps and 8Ohm speakers you could only connect say 4 to one line. If my maths is right, a speaker requiring 4W on a 100V line system will present an impedance via the transformer of 2500Ohms so you can connect quite a few of those in parallel!
Small 100V amplifiers (up to a few hundred Watts) will use a step-up transformer but for larger applications, you can use a conventional power amplifier, usually in bridge mode to produce the 100V line drive directly.
