LAN Manager (LM) is a family of early Microsoft client/server software that allows users to link personal computers together on a single network. Network capabilities include transparent file and print sharing, user security features, and network administration tools. In Active Directory if the Kerberos protocol is not negotiated for some reason, Active Directory will use LM, NTLM, or NTLMv2.
Read the full blog post: https://calcomsoftware.com/the-policy-expert-network-security-lan-manager-authentication-level/