Direct current, or DC, is electric current that flows in one direction. Batteries, USB power, solar panels, and many electronic circuits use DC.
Why electronics use DC
Chips, sensors, LEDs, microcontrollers, and most digital circuits need stable DC voltage. Even devices plugged into a wall outlet often convert AC into DC before the electronics can use it.
How AC becomes DC
A rectifier can convert alternating current into direct current. A power supply may then filter and regulate that DC so the circuit receives the right voltage.
DC is easiest to understand as steady-direction electricity, even when its voltage level changes.