Use Quick Look to view a file or folder without opening it. You can use Quick Look for items in Finder windows, on your desktop, in email messages, and other places. When you’re done, close the Quick Look window: Press the Space bar, force click again, or by click the Close.
Quick Look is a quick feature developed by Apple Inc. which was introduced in its operating system Mac OS X 10.5 Leopard. The feature was Quick Look is among the few features I missed from Mac OS X. It enables very quick preview of file by pressing Space key while highlighting it, without opening.
A QLPreviewController object, or Quick Look preview controller, provides a specialized view for previewing an item. To display a Quick Look preview controller QuickLook | Apple Developer Documentation.
Introduction to Quick Look Programming Guide – Apple Developer Those applications can include Quick Look generators: plug-ins that convert a given document from its native format into a format that Quick Look to view photos, files, or a folder without opening them. You can use Quick Look for items in Finder windows, on your desktop.