I'm not sure about whether it's supported on Windows (and why if it's not), but I think it refers to having whichever window the cursor is over being "active" by default. For example, if you have two windows open that both have a text input focused (like maybe a text editor and a browser), the more common window-management paradigm is to consider one of them "active" independent of the cursor. If the text editor is active, you need to "switch" to the browser to type into it (by clicking on it, alt-tabbing, closing the editor, etc.). An alternative way of doing things would be to have the text you type go into whichever window the cursor is over. I haven't tried out using this, but I do find this behavior noticeable when playing a game on one monitor and having a web browser open on the other. Often if something is loading, I'll switch over to the browser to read hacker news or something while waiting, but then when the game is finished loading and I switch over, I'll try to move or something and be momentarily baffled at the lack of movement until I realize that I just typed "w" or something into the browser.