We all experience different moods and stresses at different times. We can find ourselves stressed by commitments, demands, tragedies and trials of various kinds.
Perhaps it is as if a human being has an elastic band inside which feels the tension. These stresses cause a response in us. We feel 'down' or 'low' or 'depressed.' Negative patterns of thoughts about ourselves and our situation arise.
Colloquially the word 'depressed' describes these normal struggles of life, but doctors use the term 'depression' or 'depressive illness' to describe what really is a step beyond usual feelings of unhappiness or disappointment.