Mood is a measure of a child's disposition. Some infants fuss and cry a lot; others are smiley and contented babies. Some children experience their cup as half full and tend toward a positive outlook; others experience their cup as half empty and have a more negative or pessimistic outlook.
A child who's more serious or negative in mood may have a more analytical way of looking at things. It may be helpful to encourage this analytical streak in a child inclined to pessimism and negativity. When speaking to a child who's upset by some occurrence, it may help to take an observing stance and speak in a neutral tone. "Wow. Was it such an awful day for you? That's an interesting way of looking at things. It's true you didn't come in first. And yet you were the only one to come in third, and most kids didn't get any award." This kind of response may help these children broaden their perspectives. It's important not to try to fix a bad mood, though. That's an exercise in frustration that tends to land parents in a negative mood, too. By contrast, children who are often positive can easily see the upside of things; however, they may need help looking at things a little more critically, when appropriate.
The great value in the temperament perspective of Chess and Thomas is in how widely applicable and useful it is. It gives parents and therapists a neutral framework for analyzing and dealing with difficult children. Taking temperament into account empowers parents by adjusting and enlarging their perceptions of who their children are, and helps them respond to children in ways that are a good fit for their individual personal styles. It allows all infants to be "good" babies without blaming mothers (or fathers) for implied "bad" babies, who happen to be temperamentally challenging.
A temperament approach isn't a panacea. Learning about temperament doesn't transform temperamentally challenging children into easygoing boys and girls. We had tough times with Ryan all the way through high school. But knowing about temperament helped us understand and parent him better, and over the years, we noticed an important shift--the hard times weren't so hard, nor did they last as long.