This is a simple open sided box with thick enough walls that you can see what's going on inside when you use the scrubber on the Preview to see what is going on in there:
Infill occurs inside of solid walls, you can make them 100% solid, but that is a waste of expensive filament and it's going to take forever to print. So to save money, we specify a percentage of how much filament we want to use to fill the "solid" walls.
(Notice that the infill inside the walls is in a honeycomb pattern. That is set at 20%. If you set it higher, like 40-50%, you will see more honeycomb in there and less air space.)
The Support is generated where there are no solid walls to hold up the top of that box. Support is generally removed after the print, it is only put in long enough to prop up the overhang long enough for the plastic to cool and set. It has a different pattern, based on what you specify. That particular pattern is set up like this:
