Typically a speaker cable has two stranded copper conductors, covered with insulation, twisted together with fillers and sheathed with an overall jacket. These relationships are defined with Ohm's Law. The job of a speaker cable is to move a substantial amount of electrical current from the output of a power amplifier to a speaker system.
Current flow is measure in amperes.
Resistance limits current flow, and decreasing it increases current flow. Halving the load impedance doubles the load current. For instance, two 8-ohm speakers in parallel will draw twice the current of one speaker because the parallel connection reduces the load impedance to 4 ohms. But power amp outputs are rated in watts. Like the effects of shunt capacitance in instrument cables and series inductance in microphone cables, the signal degradation caused by speaker cabling is always present to some degree, and is worsened by increasing the length of the cable.