"Nature's first green is gold"
The first half of this line is a metaphor. It compares the beginning of spring and new life to the color gold. Gold is also symbolic of something that is good or valuable.
"her hardest hue to hold"
This half of the line is personification. Personification is giving nonhuman things humanlike traits. In this line it gives Nature, a nonhuman, the ability to hold, which is a human trait.
These lines are describing when things begin in Nature. Most often this occurs in Spring. The flowers begin to shoot up from the ground, and new animal babies are born. This is the time where everything is golden. Things are new, exciting, and innocent. Unfortunately, this doesn't last, which is why it's hard to hold. Things grow up and change. They gain more knowledge and lose their innocence they once had.
