Simplify the following text for a Grade 8 native English-speaking person.
Original: Mood disorder is the term designating a group of diagnoses in the Diagnostic and Statistical Manual of Mental Disorders ( DSM IV TR ) classification system where a disturbance in the person 's mood is hypothesized to be the main underlying feature . The classification is known as mood ( affective ) disorders in ICD 10 .
Simplified: A mood disorder or affective disorder is a type of mental illness where the main problem is the person 's mood .