There are many definitions of mental health. They vary on the details but one thing they have in common is an emphasis on resilience, meaning your ability to bounce back from disruptions in your life and progress towards whatever goals you set for yourself, however big or small.
If you have a high degree of resilience you tend to be positive, productive and adaptable. If you are lacking in resilience then you tend to be knocked off course easily, and can have a hard time getting back on track. In extreme cases this lack of resilience turns into depression, anxiety and other mental health issues.
Mental health is not a fixed trait, it is a dynamic state shaped by what are called risk factors and protective factors. Risk factors can include abusive family, poor health or drug and alcohol problems, while protective factors include strong relationships, a sense of community and belonging, and a sense of self-efficacy.