The purpose of a Directory Server such as OpenLDAP is to centralise user or other information. Centralising user accounts in a single Directory makes is much more simple to provide users access to desktops and servers across your network. They need just the single user account stored on the one server. User accounts are one way to start understanding OpenLDAP Directories but they can also be used to stored DNS and DHCP data as well as other configurations.
Directory Services
OpenLDAP is a major Open Source software project and the needs really arise from the growth of connectivity and the need for centralising data and authentication. The X.500 standard first defined the needs and requirements of a Directory and LDAP, the Lightweight Directory Access Protocol provided on implementation. LDAP is used in the OpenLDAP project as well as other commercial ventures with eDirectory from MicroFocus and Active Directory from Microsoft.
Understanding OpenLDAP Directories
For the purposes of the Linux Essentials exam you will just need to know about OpenLDAP as a Directory Server and not really too much detail on configuring it. If you want to setup the Raspberry Pi as an openLDAP server then these two links should help you. The first look at installing OpenLDAP and the second look at configuring servers to use the LDAP server for authentication. Both use the Raspberry Pi.
Installing OpenLDAP on the RPi
Authenticating the RPi to a Central OpenLDAP Directory
In the video we look at connecting to the openLDAP server using lat, the LDAP Administration Tool.