An alternating current or voltage is one which changes with time. The voltage goes up and down in a regular pattern -some of the time its positive and some of the time its negative. An oscilloscope (a special type of voltmeter) can show the waveform of an alternating current.
A trace can be seen on an oscilloscope- this is made by an electron beam moving across a screen. The time base controls how fast the beam is moved across the screen. This can be set by using a dial on the front of the oscilloscope. The vertical height of the trace at any point shows the input voltage at that point.
A direct current (d.c.) source is always at the same voltage, so a horizontal line is seen. However an alternating current (a.c.) source gives a regularly repeating waveform.
Measuring the distance between successive peaks along the time axis gives the time period (as long as the time base setting is known…