Windows in Spacemacs allow you to organise your buffers, splitting horizontally and vertically with multiple rows and columns. Windows can also be moved around and resized without resorting to using the mouse.
SPC w is the windows menu.
SPC w u will undo a change to the windows.
Golden Ratio - added focus for multiple windows
SPC t g toggles golden ratio mode
Golden ratio automatically makes the active window the largest in the frame. Chainging windows will resize all windows to make the new active window the largest
Windows Transient menu
SPC W . opens the Windows Transient state menu, enabling single character keybindings for window commands, simplifying multiple commands.