This is the diagram of an alternating current generator.  The solenoid is around a rotating armature shown. The axis of rotation is in green. When side A goes down side B comes up.  At the moment the current is flowing from red to blue but on a moment, that is after half a turn, the current will flow in the opposite direction.

Number of changes in the direction of current in one second, gives the frequency. In most countries the frequency of domestic current is 50Hz.

Changing voltage

Alternating current is not smooth as a direct current. The voltage depends on the rate of cutting magnetic lines of force by the solenoid.  The diagram below shows the variation of voltage as the armature rotates.

Root Mean Square voltage.

Usually when AC voltage is given it is neither the maximum nor the average voltage.  What would be quoted is called the root mean square .(rms)  This is the DC value of the AC.  In order to get the power we should use the rms value.

Maximum voltage (Peak value)  =  Voltage rms.  X  √2 .