When set to true, lets you middle-mouse-paste into the content area to load the url in the PRIMARY selection. That way you don't have to worry about whether selecting the text in the URL bar so you can replace it with the URL will clobber PRIMARY.
Only relevant on X, where there is a PRIMARY, of course. See https://unix.stackexchange.com/a/139193 for a quick description of what PRIMARY is and how it differs from CLIPBOARD.
Seems to only apply to Linux, but basically it either pastes your clipboard content into any focused text field or tries to open the clipboard contents as an URL (and falls back to Google Search if that fails).
Only relevant on X, where there is a PRIMARY, of course. See https://unix.stackexchange.com/a/139193 for a quick description of what PRIMARY is and how it differs from CLIPBOARD.