This 78018 IC is a 18 Voltage Regulator that restricts the output voltage to 18V output for various range of input voltages. So as the name suggests, the voltage regulator is a circuit that is used to regulate (fix) the voltage fluctuations in the input and therefore, makes the circuitry safer. LM7818 is an extremely popular and easy to use linear voltage regulator for generating a regulated 18V. It is a easy and simple to use voltage regulator with no external circuitry or components needed for voltage regulation (except for a few smoothing capacitors). It can provide around 500 mA of current without an heatsink and upto 2 A with an heat sink.
Note:This regulator has a ~2V linear drop-out. That means you must give it at least 20V to get a clean 18V out.