Copy & pasted directly from the manual (https://www.minibb.com/forums/manual.html#modsarray
Moderators definition. Moderators are people who can help admin in managing forums (read more in moderators section. Variable $mods=array(); in bb_specials.php defines the the array of forum/user IDs which have moderators possibilities.
means that user with ID = 9 is the moderator for forums with IDs 1 and 2, and user 996 is moderator for forum 1.
Moderators have access only to specified forum.
Please check the manual next time before you post. Usually you'll get your answer faster that way.