Part of macOS
QuickLookUIServiceQuick Look
○ Leave it
Makes previews and thumbnails of files.
What it is
Draws previews when you press Space in Finder, and thumbnails for file icons.
If you stop it
macOS starts it again straight away. Leave it.
The verdict
○ Leave it. Part of macOS, or something that starts again by itself. You never need to close these.
In Activity Monitor
QuickLookUIServiceQLPreviewGenerationExtensioncom.apple.quicklook.ThumbnailsAgent
From Procession’s guide, in the words the app shows.