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.