2ohm or 4 ohm is the measurement of impedance on the coil of a driver at it's static state(the impedance will change significantly when the driver moves) Usually the less resistance presented to the amplifier the less control at a given power, or less impedance the more distortion.


http://www.rocketroberts.com/techart/spkr.htm
http://www.slightlyaltered.com/id34.htm