This policy setting allows you to specify whether to require user authentication for remote connections to the RD Session Host server by using Network Level Authentication (NLA). This policy setting enhances security by requiring that user authentication occur earlier in the remote connection process. If you enable this policy setting, only client computers that support Network Level Authentication can connect to the RD Session Host server.
Read the full blog post: https://calcomsoftware.com/the-policy-expert-rds-require-user-authentication-for-remote-connections-by-using-network-level-authentication-nla/