An electronic component, also sometimes called a condenser. Capacitors, which come in many shapes and sizes, basically do nothing more than store electrical voltage, somewhat like a battery. They are comprised of two (or more) electrical conductors (plates) separated by an insulator. Current cannot flow “through” a capacitor without breaking it (by breaking down the insulator). Thus they do not pass any DC, but do pass AC in varying degrees by the charging and discharging of the two plates. In electrical circuits their behavior in terms of the time it takes them to store and release their voltage charge can be used in more ways than your credit card at the mall. Their use can be as simple as removing a DC component from an AC signal or short-term power for memory in electronic instruments, but they are also used in very sophisticated audio and digital circuits (as control elements). Capacitors are important ingredients in most EQ circuits, power supplies, oscillators, clocks, filters, and the list goes on and on. They are everywhere. A typical mixing board could have hundreds or thousands of capacitors.
There has been a great deal of eyebrow raising (an exercise much practiced in the audio industry) about the relative merits of different types of capacitors used in audio circuits. Many audiophiles hold that the quality of these components can have a significant impact on the audio quality (especially long term) of a device. This type of thing is one of many that may separate more expensive equipment from the cheap stuff.