What macOS feature lets you preview a file without opening it?

The macOS feature that lets you preview a file without opening it is Quick Look. In Finder, selecting a file and pressing the Space bar opens a large preview without launching the application normally associated with that file.

Apple introduced Quick Look with Mac OS X 10.5 Leopard in 2007. It was designed to make file browsing faster: users could inspect a photograph, PDF, text document, presentation, audio file, or video without waiting for a full application to open. Depending on the format, the preview may support scrolling, playback, or other limited interactions.

Quick Look is different from Preview, the separate macOS application used for more extensive work with PDFs and images. Finder may also show small thumbnails or a preview pane, but those are not the same as invoking Quick Look. Quick Look can handle many common formats, and additional plug-ins or system support can extend the types of files it understands.