Add power to activate. Active components require a power source to affect a circuit.
As the name would imply, a voltage regulator maintains a consistent voltage, below the supply voltage rail. They come in several form factors, and are available for both positive and negative voltage, but all essentially do the same thing: dump voltage off in the form of heat to reduce the supply voltage down to a lower level. This is useful for circuits that have IC chips with different maximum supply voltages. For example, if you have a circuit that uses a hex inverter with a maximum supply voltage of +5V and an op amp that needs +12V, you would use a 5V regulator to step the 12V supply down, and power the inverter with the stepped-down voltage.
Big regulators can produce a lot of heat, so it might become necessary to use a heat sink to protect the circuit.