The load value (in ohms) that the speakers present to the amplifier - the amount of resistance to the current flow. Low-impedance subs (less than 4 ohms) or multiple subs wired in parallel can cause problems with amplifiers which are not designed to operate at lower impedances. This is especially important if you want to bridge your amplifier and run multiple subs. Lower-impedance subs are suited to mono amps, which are usually designed to handle a 2-ohm load (or even lower, in some cases).