Consider a scenario where you have a vim window that has been split horizontally into two viewports. You'd prefer the top one to be on bottom and the bottom one to be on top. You want to swap them.
If you are currently focused on the top viewport, then tell vim to move that
viewport down with CTRL-w J. As you might guess, moving the bottom
viewport up can be done with CTRL-w K. J and K mean down and up in
other contexts; vim is consistent with their meaning here.
Viewports of a vertical split can be swapped in the same sort of way. Use
CTRL-w L to move the left viewport to the right. Use CTRL-w H to move
the right viewport to the left.