Joy is one of those words that is hard to define, but Merriam-Webster tries. Two of the definitions for Joy are: state of happiness or felicity and a source or cause of delight. Well they tried. I think joy is one of those things you just have to experience and you recognize when you are experiencing it.
Joy is the wonder of the miracle of life at the birth of a baby.
Joy is the excitement as couple start out in life together at a wedding.
Joy is also (and this is my favorite) that unexplainable and unstoppable burst of happiness that starts as a little giggle down deep in your soul and then explodes all over everyone.
Joy can also be that feeling of quiet contentment that feels like a warm blanket wrapped around you.
To experience joy you have to be open to it. Singing also helps.