Tags are a relatively new thing on journals. The term 'tag' when writing with someone in an RPG comes from either playing tag - the chasing game where you touch someone and say 'tag' and they have to chase you - or from tag-team wrestling.
When writing via AIM or some other IM, one writes 'tag' when one has finished and it's the other person's turn. If one is writing in a journal, the email notification to ones writing partner can act as a tag, though I'm usually IMing them as we write because what we do tend to be plot heavy. Each game has its own accepted courtesies. If you are worried about getting it wrong, why not contact one of the moderators and ask what is expected of you?