User directories are what Jira uses to store information about users and groups. A user directory is backed by a user repository system, such as LDAP, a database, or a remote user management system, such as Atlassian Crowd.
You can have multiple user directories in Jira. This allows you to connect your Jira instance to multiple user repositories. For example, you can have an LDAP directory for your internal users and the Jira internal directory using the database for external users. An example is given in the following screenshot, where we have three user directories configured. The first user directory is the built-in Jira Internal directory running on the Jira database. The second user directory is connected to the Microsoft Active Directory (Read Only) in read-only mode. The last user directory is connected to Atlassian Crowd, user identity management software...