What exactly does this mean? I don’t remember seeing it there before, doesn’t mean it wasn’t though. What’s better? Less time or more time?
See this thread Remote Web Playback Buffer
I think it was added to the Version 2018.11.20.2224 DVR release since he mentioned adding it to the beta pre-release on Oct 10th
Appears no matter what you enter for Client Buffer time, it doesn't persist (doesn't appear to save the setting) and goes back to the default of 30 seconds. I also checked the dvr ip http://x.x.x.x:8089/settings and don't see that setting anywhere in the settings json data.
Works for me. The setting is stored in browser local storage.
That explains it, so it's tied to the browser that uses it, not the DVR.
I use Private Browsing in Firefox (like chrome incognito mode) so it doesn't store cookies.
Perhaps because the buffering is in the client, not the server?
Why would you use private browsing to your own DVR 
Yes and Yes. Thanks for the difinitive answer. Wasn't in the Readme !
I use Private/Incognito browsing for security and privacy as a default on all my devices and don't even think about it until something like this occurs.