Actually I wonder whether spectrograph is a specific enough description.

This one shows 3 dimensions (time, frequency AND intensity) - so its not one of those that looks like an oscilloscope. It looks more like an ultrasound scan (though of course it isn't any such thing).

The importance is in being able to see the relative intensities of a range of frequencies at each single sampled instance - ie you see the fundamental & the harmonics as bands of light. The Y axis is at the right hand side of the screen, and this is where the sample appears - it then moves toward the left of the screen as the next appears - building a moving picture of the last x seconds' worth of sound.

As it tracks the input you see the changes with time - so you can see vibrato (and wobble) - and you can see where your voice is strong & where it is not so strong. You can see where you have lots of harmonics (ie a rich tone) and where your tone is thinner. You can see if you are going a bit flat on a held note. All manner of things...... very useful (and impartial) feedback.

You can even see the claw-clackings of the dog as it wanders past on the laminate floor (must remember to clip those claws).

Kris