Part of macOS
quicklookdQuick Look
○ Leave it
Makes previews and thumbnails of files.
What it is
Draws previews when you press Space in Finder.
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
quicklookdQuickLookUIHelper
From Procession’s guide, in the words the app shows.