From Online Manual
A moderator is assigned to a board by an administrator. Unlike a Global moderator, a moderator does not have permission to moderate outside their assigned board(s), and is a regular user outside the board(s) (s)he moderates.
Moderator is a special membergroup, which is located in the permissions section of the admin panel. An administrator can also use this to change the permissions assigned to moderators.
To make a member a moderator please see Moderators - How do I assign members as local moderators.