When a person's moderator application is approved that stated person is put on trial for one week. Note that the trial can be extended or shortened by a Head Admin. Staff Trainers may extend a trial with a Head Admin's approval but cannot shorten it. If the trial moderator passes his trial, then he will move onto to be a Moderator. If he fails his trial, then he will be released. Trial moderators have some moderator commands but not all.
You can apply to become a Trial Moderator by filling out this application.
If you are a trial moderator and you would like tips for passing your trial, then check out Emily's tips for doing so.