|Grove of Olive Trees|
What it means is pretty simple. It means one who is in good spirits (being cheerful), keeps you well and healthy. Being down and depressed is a slow death.
I've actually seen this too many times with people I know. I've witnessed so many times a bitter spirit lead to heart attacks, cancers, and other ailments. My theory on this is that the stress and anxiety triggers an autoimmune attack, leaving the body vulnerable to other illnesses.
The verse is insightful because it certainly suggests that you can chose to be cheerful or gloomy. In short, while adversity may choose us, (although difficult) we can choose to be gloomy or cheerful about it.
I selected olive trees as my picture on this post because they certainly look cheery to me. Some translations of the verse say that good spirits is like good medicine.