SMPS IS Switch Mode Power Supply. The primary function of SMPS is to convert the alternating current (AC) power available in homes into direct current (DC) required for a computer system. In desktop computers, you will find a metal box in the corner of the CPU case supplies power to various components in the CPU box. The power supply converts 115-230 volt AC into DC that is required for computer components to work.

AC stands for alternating current. It is an electric current whose magnitude and direction vary cyclically.
DC stands for direct current. It is considered as the constant flow of electrons in a single direction from low to high potential. Computers use DC power. Because of the advantage of alternating current over direct current in transforming and transmission.