When you use your Mac, the general rule is that you can work in only one window at a time. But you can get things done in your ‘background’, or inactive, windows even without clicking one of them to bring it forward.
Close or rearrange background windows. You’ve opened several Finder windows, and now your screen is getting cluttered. Click a background window’s close (red) or minimise (yellow) button, and you’ll immediately banish that window without deactivating the one you’re working in. By holding down the c key, you can drag a background Finder window by its title bar to reposition it, all while keeping whatever you’re working on – a Word document, for example – active.
Scroll in a background window. Say you’re comparing two multi-page PDF documents in Preview. You don’t have to keep clicking back and forth in the windows to move through both of them. If your trackpad lets you scroll by dragging two fingers, or if you have a mouse with a scroll ball or wheel, hover over the area of the background window you want to scroll, and … scroll.
The file in the background window will move, but your Mac will keep the focus on the one in the foreground.