A person who is calm and peaceful and doesn't get upset easily is an example of a person who is mellow.
Coffee that has a smooth, calm flavor and that isn't harsh is an example of coffee that would be described as mellow.
A mellow friend; a mellow conversation.
A mellow voice; mellow light.
If you were prone to swings of temper when you were younger but have become calmer as you age, this is an example of a situation where you mellow.
A mellow fruit.
A mellow wine.