A characteristic of electricity is that it is not a primary energy freely present in nature in remarkable amounts and it must be produced. Production is carried out in power stations (also called “power plants”)
Most power plants/station contain one or more generators, a rotating machine that converts mechanical power into electrical power. The relative motion between a magnetic field and a conductor creates an electrical current.
So source can be different to move generator:
How does frequency come, as we know now that electrcity is produced by conversion of mechanical movement to electicity using magnetism phenomenon.
So when a conductor rotates in magnetic field, the magentic field of lines are cut and current is induced in conductor. Since magnet has lines of field in north to south(uni-director). So conductor will move back and forth in this uni-direction. So when it cuts all the lines, then there is max current induce, and then slow goes back no lines cut then current is zero. Then conductor comes back in opposite direction so reverse current is induced.
So this is like sine-wave, current goes to maximum then zero then negative(other side) maximum. We have rectifier to do stuff, but this is what frequency is in electric current.