Windows in Spacemacs allow you to organise your buffers, splitting horizontally and vertically with multiple rows and columns. Windows can also be move around and resized without resorting to using the mouse.
The windows menu is
SPC w u will undo a change to the windows.
Using the golden ratio mode automatically increases the size of the active window, which is very useful when you have multiple windows open.
Toggle golden ratio mode using
SPC t g
Windows Transient menu
Use the Windows Transient state menu
SPC W . to re-arrange windows multiple times.