raise window underneath the current active one
If the window you want to raise belongs to the active app of the frontmost window, then you can use ⌘` to bring the next window forward.

You can then use ⇧⌘` to bring back the previous window. (This combo also walks backwards through the apps windows.)



If the window you want to raise does not belong to the active app, but a different app altogether you can use ⌘tab and then ⌘tab again to get back to the previous app, assuming it hasn't been closed.

⇧⌘tab can be used to walk backwards in the app switcher.




⌘ is the command key.
` is the Grave Accent, and on a US keyboard is just above the tab key.
⇧ is the shift key,