The impedance is the electrical resistance of the Voice coil(s). Amplifiers are rated at different ohm loads and put out different amounts of power at different loads. In most cases the amplifier will put out more power, the lower the ohm load. BUT... they are only rated down to a certain load. Most 2 channel amps are rated at 2ohm on each channel and 4ohm bridged. Some mono "1ch" amplifiers are rated at 1ohm.
RMS is the constant power that a subwoofer can handle or the constant power that an amplifier puts out.