Mental health is about how we feel, think and behave. Nobody feels happy and thinks positive thoughts all the time.
Often when we find ourselves feeling very low or stressed and anxious we tend to think it's just part of life and don't do anything to help ourselves.
There are times when it's natural to feel sad or anxious - when you've suffered a loss, or if you have to adjust to sudden change.
But if your mental health is basically good, you're likely to cope better in a crisis and recover more quickly.