"RedirectLiveToWWW" is a URL rewrite rule in order to make non-www URLs (such as mysite.com) end up as the www URL (such as www.mysite.com) in the browser. The site license needs to have the www. prefix on the production URL for this to work, and of course the DNS must have a www.mysite.com entry pointed to the site IP. The value of doing this is in SEO practices (you should ideally only have one URL format), and possibly out of necessity (your SSL cert may only be valid for www. format). The reverse of this is also possible (redirect www to non-www), but not as common. If your site is (or is going to be) a "mega" site, then the www format is best.
The "RedirectLiveToWWW" setting / AppConfig that has been in versions previous to 10.x.x has been replaced with a URL rewrite rule in the web.config file to guarantee the functionality is applied across every request. This feature can now be enabled and disabled by setting the "enabled" attribute of the "Ensure www subdomain" rewrite rule to "true" or "false".
NOTE: preFIX sites will not have access to all items normally found in the root. Instead of editing the web.config, you will need to locate the RewriteRules.config file: /config/RewriteRules.config