In this series of Side Image and Down Image scans on my Blog I’m trying and teach you what you are looking at while scanning with your Side Image and Down Image. This one will be of bottom composition.
If you notice the dark color in the screen shot is a mud or muck bottom. The sonar signal is absorbed in the soft material where the hard bottom in this case gravel and rock is hard and the sonar signal returns very quickly thus a brighter color on your screen.
Hope this helps you understand units.