it means that when all things start out as gold since it is natures first hue for example a child, everyone is a child at some point and that “nothing gold can stay” is saying that no one is a child forever and that you should cherish your childhood fully and make the most out of it. As the poem mentions “dawn goes down to day meaning” that nothing is eternal. Also the part “her early leafs a flower but only so and hour then leaf subsides to leaf so Eden sank to grief”. This means that flowers only bloom their fullest for only a short while then its bloom slowly starts to subside to nothing. Meaning that all things wonderful can’t stay Which is why Eden is sad. The part “Nature's first green is gold,
Her hardest hue to hold” means that the best thing in life represented by gold can’t last forever represented by the second line.