Positive psychology is the study of happiness. Psychology traditionally focused on dysfunction: people with mental illness or other psychological problems and how to treat them. Positive psychology, in contrast, is a relatively new field that examines how ordinary people can become happier and more fulfilled.
Optimists have a tendency to make lemonade out of lemons, and to then see the glass as half-full rather than half-empty. It's an admirable quality, one that can positively affect mental and physical health.