A trial separation is an informal splitting of a couple. Unlike legal separation

Legal separation

Legal separation is a legal process by which a married couple may formalize a de facto separation while remaining legally married. A legal separation is granted in the form of a court order, which can be in the form of a legally binding consent decree...

, this is purely a personal arrangement and does not require legal proceedings. The couple can choose later to go through with a divorce

Divorce

Divorce is the final termination of a marital union, canceling the legal duties and responsibilities of marriage and dissolving the bonds of matrimony between the parties...

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.