A proxy list is generally an extensive list of public HTTP/HTTPS/SOCKS proxy servers that are not password protected. Most often HTTP/HTTPS and SOCKS4/SOCKS5 proxies are kept in separate lists, and those lists must be tested with a proxy checker frequently. For many years it was hard to find proxies and they could really only be found by doing port scanning and other not-so-legal things, so owners of lists would usually keep them private and only share them for a good price. But recently, especially in the last decade, people have begun sharing proxy lists because there are so many more open proxies on the internet, you can find thousands of public proxy lists just by searching Google.
However, there are also premium paid lists of public proxies available online that will have open proxies that many other free lists will not include. These are often filled with proxy servers found some-what recently via port scanning or by other means such as the installation of freeware/malware on computers. Free lists easily found with Google, on the other hand, are generally just old proxies scraped from older proxy lists on Google search results and tested with a proxy checker before being uploaded as a new list. You can see why free proxy lists are often very stale and contain many dead or unstable public proxies. Premium public proxy lists are often far superior and will have much better proxy success rates. Check out Proxy-List.org to see a site selling access to a tested public proxy list.
If you are totally lost or just want to learn more about proxies go here. If you are not interested in public/open proxies and would like premium private proxies, check out any of the Proxy Providers listed on this site or use the Compare Pages to find just what you need!