What is a spectrogram? What variables are on the x - and y-axes?

Definition

"Spectrogram is a visual representation of the strength or loudness of a signal over time a various frequencies in a particuar waveform".

Explanation

spectrograms are used to display frequencies of sound waves produced by humans, machinery, animals, whales, jets, etc., as recorded by microphones. Time variable is plotted on x-axis and frequency is plotted on - axis.