Mental health is a state of well-being in which:

A person understands his or her own abilities, can cope with the normal stresses of life, can work productively and fruitfully, and is able to make a contribution to his or her community.

Both physical and mental health are the result of a complex interplay between many individual and environmental factors

Source: https://www.ccohs.ca/oshanswers/psychosocial/mentalhealth_intro.html

Examples of mental health problems & disorders: 

Depression

Eating disorders

Anxiety

Bipolar disorders

Burnout

Schizophrenia and other psychotic disorders

Sleep disorders

Substance abuse

|  Source: https://www.ccohs.ca/oshanswers/psychosocial/mentalhealth_intro.html

But wait there is more… Mental Health is:

More than the absence of mental disorders.

An integral part of health; there is no health without mental health.

Determined by a range of socioeconomic, biological and environmental factors.

|  Source: https://www.who.int/news-room/fact-sheets/detail/mental-health-strengthening-our-response#