LDAP stands for Lightweight Directory Access Protocol and is sometimes known as active directory. For more information about how to setup LDAP for the first time, you can view the LDAP Setup Instructions

When a user is removed from LDAP, the system caches their password. Once users are no longer tied to a role that uses LDAP, the cached password becomes their permanent password. This means that the last password they used to login will still be valid and the user can login as normal.