LAN Manager is a network operating system originally co-developed by IBM and Microsoft. It supported the Server Message Block (SMB) protocol and, at the time of its deployment, it allowed for a very robust mode of communication within an local area network (LAN). It has since been replaced by other modes of communication within an LAN.
Techopedia explains LAN Manager
At the time of its release in the late 1980s, LAN Manager was actually quite visionary. Microsoft's release of LAN Manager in 1990 showed even more vision with its support for TCP/IP. LAN Manager became rather infamous as the years progressed as its mode of authentication became rather vulnerable to brute force and rainbow table attacks. Microsoft then replaced the LAN Manager hash with NTLM, but some vulnerabilities still remained. Microsoft has since gone on to support the Kerberos authentication protocol, but it still supports of LANMAN and NTLM for legacy systems.