In audio electronics, the usage of Ohms refers to the amount of resistance that a circuit has to current; it is an inversely proportional ratio, that is, with a lower Ohms rating, a higher current flows through the circuit and load.
In the case of speakers, an 8 Ohm speaker can only “accept” a load current that is 1/2 as great as a 4 Ohm speaker; likewise, a 2 Ohm speaker can handle a current that is double that of a 4 Ohm rated speaker.
Ohm’s Law is the clearest illustration of this principle, and if you know two of the three terms (the Voltage used in a system, and the current), you can calculate the resistance (V=I/R, where Voltage is equal to Current divided by resistance in Ohms).
This is a common calculation in determining the load of a speaker or driver, in order to properly match the amplification being delivered from a power amp. If your amp is switchable, it will decrease its current when the resistance is increased. Usually smaller speakers can handle lesser currents, so they will have 8 or even 16 Ohms. Larger reinforcement speakers, subs, etc., can often handle 4 or even 2 Ohm loads, given proper amplification. More sophisticated “pro-level” power amps have this switch on the rear panel, where it will say something like “250 Watts (Voltage x Current) @ 4 Ohms, 500 Watts @ 2 Ohms.” Usually, amps and speakers use multiples of 2-that is, 2, 4, 8, 16. I have not seen systems with 6 Ohm ratings, though in electronics they do exist (not so much in audio applications).