set up a base LDAP directory structure (OUs/Organizational Units, I only use 3 OUs: system, users and groups)
an admin user in the LDAP directory (mine is admin directly at the base of the LDAP directory)
(optional but recommended) a so-called bind user in the LDAP directory (unvprivileged account that can only list/read users/groups) (mine is bind under the system OU)
(optional) groups to map users to their roles (e.g. only users in access_jellyfin are allowed to login to jellyfin)
actual user accounts, member of one or more groups if needed
When you login to an application/service configured to use the LDAP authentication backend, it connects to the LDAP directory using the bind user credentials, and checks that the user exists (depending on how you configured the application either by name, uid, email…) , that the password you provided matches the hash stored in the LDAP directory, optionally that the user is part of the required groups. Then it allows or denies access.
There’s not much else to it:
you can also do without the bind account but I wouldn’t recommend it (either configure your applications to use the admin user in which case they have admin access to the LDAP directory… not good. Or allow anonymous read-only access to the LDAP directory - also not ideal).
slapd stores its configuration (admin user/password, log level…) inside the LDAP directory itself as attributes of a special entity (cn=config), so to access or modify it you have to use LDIF files and the ldapadd/ldapmodify commands, or use a convenient wrapper like the ansible modules tools used above.
once this is set up, you can forget LDIF files and use a web interface to manage contents of the LDAP directory.
OUs and groups are different and do not serve the same purpose, OUs are just hierarchical levels (like folders) inside your LDAP tree. groups can contain multiple users/users can have multiple groups so they’re like “labels” without a notion of hierarchy. You can do without OUs and stash everything at the top level of the directory, but it’s messy.
users (or other entities) have several attributes (common name, firstname, lastname, email, uid, password, description… it can contain anything really, it’s just a directory service)
LDAP is hierarchical by nature, so user with Common Name (CN) jane.doe in OU users in the directory for domain example.org has the Distinguished Name (DC) cn=jane.doe,ou=users,dc=example,dc=org. Think of it like /path/to/file.
to look for a particular object you use filters which are just a search syntax to match specific entities (object classes) (users are inetOrgPersons, groups are posixGroups…) and attributes (uid, cn, email, phonenumber…). Usually applications that support LDAP come with predefined filters to look for users in specific groups, etc.