Ohms ImpedanceSpeakers and amplifiers normally have a rated impedance. (Most speakers are around 6-8 Ohms)
The lower impedance the more current is used from the amplifier making it more troublesome for an amplifier that is not designed to cope with a lower impedance to drive.
A higher impedance is easier for the amplifier to drive and so is less of a worry.
Some amplifiers have an impedance selector switch to match the speakers better
Ohms: A measurement of resistance. Speakers use ohms to measure their impedance
The lower impedance (or amount in ohms) the harder the speakers are to drive.