A trial separation is an informal splitting of a couple. Unlike legal separation, this is purely a personal arrangement and does not require legal proceedings. The couple can choose later to go through with a divorce or can choose to stay married.
A trial separation may allow the spouse who wants the divorce to experience some of the feelings of being separated without making a final decision to divorce. The main advantage of a trial separation, of course, is that it's easily reversible. You can try it for a while, go through counseling, and then reconcile, or you can try it for a while and then proceed with divorce.