It is usually very important to transfer power from one stage of an electronic device to the next. To transfer the maximum amount of power, the output impedance of one stage must be the same as the imput impedance of the next stage.
Impedance is the resistance of a circuit to alternating current. If a small impedance is connected to a big impedance, then the power that can pass through the connection is limited by the larger impedance.
Impedances matched by transformers are used for high power circuits. A transformer converts alternating current at one voltage to another voltage, however the power remains the same, except for conversion losses. The side with the lower voltage is attached to the low impedance, because more current can flow through the lower resistance. The side withthe higher voltage goes to the higher impedance, because more voltage can get through the higher resistance. The most visible examples are the power transformers used to distribute power from high impedance transmission lines to low impedance retail use.
Resistive impedance matches are the easiest to design. They limit the power deliberately. They are used to transfer low-power signals such as unamplified audio or radio frequency signals in a radio receiver. Almost all digital circuits use resistive impedance matches, usually built into the structure of the switching element. See resistor.
Some special situations, such as radio tuners and transmitters, use tuned filters to match impedances for specific frequencies. These can distribute different frequencies to different places in the circuit.