Transparent Windows is the name of a little Gnome Shell extension that obviously makes all windows transparent. You may want to have a look behind active windows, get info from two or more full screened windows and work on the active one, or just do it for the effect and eye candy side of things. For whatever reason you may need to have transparent windows of adjustable opacity, this is a quick way to get there.
You can either set the transparency from GS extensions preferences tool, or just use the quick set pop-up by clicking the extension area on the top panel (although this is a bit confusing as it works on the “off” position for me).
To install this extension, simply press the below button and then turn the switch to the “on” position. A preferences button should appear next to the switch if your refresh the newly opened tab.