Nature's first green is gold,
Her hardest hue to hold.
Her early leaf's a flower;
But only so an hour.
The “first green” of Spring is precious, as valuable as “gold,” bursting with light and promise. 
 
 We can interpret this line both literally and figuratively. 
 Literally, the buds of willow and maple trees appear gold before they turn to green leaves. The line gives the poem a timeframe: the very beginning of spring. 
 Figuratively, gold is a symbol of wealth and currency. Frost implies that the most valuable aspect of nature is the moment before buds turn to leaves. Perhaps this is a metaphor that the most valuable time in a life occurs after birth, and before maturity sets in: the “early spring” of a lifetime.
Nothing Gold Can Stay