You can set the action to do different things in System Preferences, but the default action is to show application windows if I'm not mistaken.
Try opening 2 separate windows (not 2 tabs) in a web browser and then open a Finder window. If you swipe 3 fingers down, you will see that only the windows for the currently active application are shown.
If the current application was the web browser, only the 2 browser windows will show and if Finder was selected, only the Finder window will show. This function is useful if you are working in an application with many windows and only want to show windows from that app instead of looking at a jumble of stuff on Mission control (3 fingers swipe up) which shows windows from every open application.