With a multiple speaker hookup, it is necessary to consider impedance. This is the electrical resistance of the voice coils of the speakers. The lower the impedance, the more electrical energy is applied to the speakers. Excessive heat at either point can cause damage to the sound system. Impedance is how much a device resists the flow of an AC signal, such as audio.
Impedance is similar to resistance which is how much a device resists the flow of a DC signal.
Both impedance and resistance are measured in ohms. Hold a piece of stiff cardboard out at arm's length and wave it slowly back and forth. Now do it faster. The same thing happens with cones and even diaphragms to a lesser degree. This is physical impedance - mechanical resistance in technical terms.