In a word, it’s resistance—opposition to the flow of electrical current (representing the audio signals) through your copper cables to the speaker drivers. It’s a kind of electrical "friction" to the movement of electrons through the copper. We measure this electrical resistance in "ohms," named after Georg Simon Ohm, the German physicist who discovered the phenomenon and formulated "Ohm’s Law."
You might worry about running 30 feet or more of cable from the receiver at one end of the room to the speakers at the other end, but think about this: The voice coil of the woofer (the bass driver) contains more than 100 feet of fairly thin wire, much thinner than your speaker cables. It’s that thin wire in the woofer voice coil plus the wire in the tweeter’s voice coil (probably 20 feet or so) and the wires in the crossover that comprise the speaker’s impedance. With most domestic speakers, that’s nominally about 8 ohms or, in some designs, 4 ohms. The 'nominal' refers to an average impedance since the actual impedance varies with each frequency.
There isn’t any way you can lower the impedance of your speakers—that’s set by the designer and the voice coil windings. But you don’t want to waste power by using speaker cable that’s too thin. If it’s really thin (22-gauge or thinner), it can actually change the sound of the speaker. Taking a conservative view, you can keep your speaker cable resistance to 0.2 ohms or less by using 12-gauge cable in all domestic installations, even if your cables are more than 50 feet long. For lengths of 25 feet or less, 14-gauge cable is perfectly fine. There’s no audible benefit to using thicker cable, because the resistance of your cables is already insignificant, well under 5% of the speaker’s resistance.
Read the next tip: "Subwoofer Placement Tips"