These lines reflect the nature of immortality of beauty. A thing of beauty is a source of joy and delight forever. The poet talks about the presence of beauty in the imaginary stories  people had made about death and the lovely stories he has heard.  Keats here says that the beautiful things are like endless fountain pouring on us from the heavens, which donate the very truth about the laws of nature that God himself has made these beautiful things so that one might derive happiness, calmness and peace in life.