(For educational purposes, this article is written for the average player, and does not go in extreme depth. Also, certain situations and externalities involving impedance are left out for the purpose of simplicity.)
If you're in the market for a pedal, you've more than likely noticed all the factors that go into a pedal's quality. The construction of the body, be it plastic, aluminum or steel; power sources and battery life; ease of use, programmability, and, of course, cost. However, many players notice that a pedal board, especially one with many components is often very noisy, or that their tone seems to be sucked away by the pedals. One factor that the average consumer looks past (bypasses, perhaps?) is the impedance of the pedal, and whether or not it is true bypass. A high or low electrical impedance level in a pedal can make or break a tone, regardless of what type of effect it is.
In the simplest terms, impedance is the amount of resistance to an AC signal which a pedal puts up when the signal travels through it's circuitry. It's similar to resistance, which is also a measure of force against a current, but unlike resistance, which is force against a direct current, impedance factors in alternating currents and is not static; it's based off the input voltage, meaning pickups play a large role in manipulating how a pedal interacts with a signal. It's a complex interaction between the components in the guitar, the pedals, and the amplifier.
The generally accepted favorable impedance levels for a pedal is a higher input impedance and a lower output impedance. When the impedance is close to the rating of the pickup, the ratio is closer to 1:1, so the signal is mostly preserved. A higher input impedance results in a lower signal decay, allowing a more sparkling sound because the entire audio band is represented. Having a low output impedance allows the signal to be easily matched by a (hopefully) high input of another pedal.
Many pedals feature a true bypass feature: the signal, when true bypass is on, enters from the input jack and goes immediately to the output. No chance of signal decay because the circuitry is bypassed. Herein lies the problem: no impedance. When the signal is only traveling a short distance, a true bypass pedal can be beneficial. When traveling long distance, it can actually hurt your tone.
Imagine a situation where you have one pedal. The cable between the guitar and the pedal, as well as the pedal and the amplifier, are 25 feet long, totaling 50 feet. Without true bypass, the signal has to travel 50 feet. Not a huge distance, but enough for signal to decay. Now, with a non-true bypass pedal with a good output impedance, the signal is buffered on the way in and out, meaning the signal is equivalent to one which travels 25 feet. Half the distance, half the decay. So while true bypass is considered to be a good feature, it can at times hinder your tone.
So there's a small explanation of electrical impedance. The subject itself is vast and complex, but a basic understanding can help anyone to make a good purchasing decision. Be wary of advertising claims; be sure that what you're buying is going to preserve your tone as much as you hope it is.