In I3-wm 4.3, container and windows is a little confused and complex. I try to give a easy understand introduction
First thing you need to understand is that I3 is tiling window manager which means each application couldn't have overlap and only are allowed to put into each "container". You could image you screen as a excel sheet which have many cells. And your application could only stay in cells.
Second, your screen is just a container.And it is possible to divide it into many smaller containers. And these smaller containers are "equal" which means they have to sit side by side (horizontal) or top by top (vertical). For example, you could dive screen into three equal rectangles side by side. But how about if you want two big rectangles and two small rectangles? Yes, you could divide one big rectangle into two small rectangles.From there you could understand that container is physical cells on your screen and you could use horizontal/vertical arrangement to organize them.
Third is windows. Container is physical screen and you could put several windows into one container. But all the windows are same size with container and they are face to face sit. You could use "stack" or "tab" to switch.