Down view is a just a downward view below the boat. On side view, you'll see a line in the center with black on both sides, that is the water column below your boat (down view). The thicker that is, the deeper the water. Everything to the left and right of that is the side view. That is everything that is out of the down view beam.
The width of your downview is dictated by the depth of the water. The side view is set by the user to how far out you want to see. Typical setting is 75-80' for better picture but you can set it up to 300' to either side. Distance may vary depending on unit. SO, say you are running at a depth that allows 10' wide down imaging. In side imaging you would be looking at 150' width (or whatever you set it at). You can also eliminate the water column in side view as well as just view right, left or both.
With DI you would have to drive over the top of something just like 2d to see what is below. With SI, you can keep a distance and not spook the fish and look for fish and structure. There is actually a huge difference between the two.
In this picture, I have both DI running on left and SI running on the right in split screen. On the right side of the screen you see a large object. That was picked up to the right of my boat. If you look closely at the left edge of it, the corner is just crossing into the water column below the boat and is all that I would see in DI. I had passed this thing 100 times and never saw it until because I had never passed directly over it. It took the SI to show me that it was to the right of my boat. If you look at the left side (di) just above the 4.1 MPH covering the DI screen, you'll see a little bump. That was how much of the object was in the DI beam and all you would have seen without SI, given the boat position.
Youtube SI videos and you will get a good understanding on how it works. It is amazing. I've found sunken boats and underwater bridges with amazing details with mine.