A moderator, aka editor, is a helper who takes on some of your load in managing your site. They can approve or reject pending topics, edit or delete topics, or edit or delete posts on topics. There are two types of moderators: site-wide moderators and forum moderators.

Forum moderators only have powers in specific forums of the site which they're considered specialist experts in. To add a forum-specific moderator/editor, edit the forum which you want them to be moderating and write the member's username into the "Moderators" box. If you're adding moderators to a bunch of forums, use the bulk edit forums page at Admin Panel -> Forums -> Bulk Edit Forums so that you can enter many at a time. You'll notice that the members you've made forum moderators have moved to the "Moderators" usergroup automatically, this can be changed/stopped with the Admin -> Members -> Member Settings -> "Default Moderator Usergroup" option. If there are any site-wide powers that all moderators should have, you can configure the moderators usergroup at Admin Panel -> Members -> Manage Usergroups -> Moderators.

A site-wide moderator has moderating powers everywhere. If you want all moderators to be site-wide, go to Admin Panel -> Members -> Manage Usergroups -> Moderators and check the boxes for allowing them to edit and delete everything. Then you can either directly assign people to the moderators group by editing their profiles, or list them under particular forums by editing the forum moderators list. Notice that since they've been granted site-wide powers it doesn't really matter what forums they're listed under, but topic them under the forums they frequent most often can be useful for keeping track of how well each forum is being monitored.

All moderators have access to the moderator control panel. A forum moderator will find this option in the editorial controls for their forum, a site-wide moderator can find a link to the moderator control panel at the bottom of every page (in most themes). The main function of the moderator control panel is to allow moderators to validate or reject pending topics. There's also a moderator whiteboard, where all moderators and administrators can write notes for the others to see. Then there's a reports tab where they can review and resolve issues reported by users, moderator notes about users, logs of past moderator actions, a utility to search IP addresses to uncover people who use multiple accounts, and a flagged posts page in the event that any terms have been set to flag posts on the posts settings.