When we talk we emit sound waves that are transmitted to our ears. The same occurs with analogue transmission using computers. Analogue signals are sent over phone lines and other lines that operate using sound. There is no break in the signal and the movement along the Sine wave never stops. The waves fluctuate a certain amount of times per unit of time, and this fluctuation over time is called the frequency.
Frequency is measured in hertz, which means cycles per second.
The more fluctuations, the higher the frequency. Higher frequency translates into higher notes. Low frequency produces low, deep notes. The signal also fluctuates in loudness. This loudness is called amplitude.