John Keats, in this poem, wishes to tell us that a beautiful thing will never decrease in loveliness (A thing of beauty is a joy forever, its loveliness increases). He tells us that unlike any other thing that we find beautiful, it's only the simple things in life that will give us perennial joy. They will never cease to appeal to our senses, and will always be there to calm us, and be the ray of light in an otherwise gloomy and troublesome world. Keats says that simple things like rills,daffodils, fair musk-rose blooms, a sound sleep, the Sun, the Moon, the cool shade of young and old trees will be a constant source of joy. He goes on to say that it is our love for these things, and the joy these have in store for us, that allow us to forget the misery of life; the pain of life. They are like a 'flowery band' that we we have wrought that binds us to the Earth, and which gives us the will to live, and to go from one day to another. He also says at the end, that stories (something we all love, whether it be as a child, or an adult) of the mighty who are no more, and stories in general also give us much joy.
The concluding lines tell us that it seems that Things of Beauty are like an eternal fountain sent to us by God from Heaven. It is, as if, God is sending us these things to sustain our joy, and make us happy even when we are in our darkest times.