This is one of four methods to establish Divorce. If a Divorce is pursued on this basis it allows the person raising the action to obtain a divorce before the normal 1 or 2 year separation ground.
A wide variety of behaviour can constitute unreasonable behaviour including physical abuse, mental abuse, financial abuse (one spouse controls the other by restricting how money is used), gambling, lack of practical support, alcohol / drug addiction. There is no definitive list of what is unreasonable but these are common examples.
If the behaviour does not quite constitute unreasonable then you should look to a Divorce on the grounds of 1 or 2 years separation.