The type of current flow - in this case - when the current flows in one single direction.
A flashlight with a battery uses a direct current.
AC stands for alternating current, when the current periodically changes direction.
In Northern America and Western Japan, this usually happens 60 times per second, or 60Hz / hertz.
In Europe, the UK, East Japan and most of Australia, South America, Africa and Asia, the current changes direction 50 times per second, which is 50Hz.
Power supplied to homes and businesses uses AC supply.