The poem starts by asserting that the first "green" of nature is "gold." At first this might be confusing—how can something that's "green" also be another color? But the "green" here refers less to actual color and more to the idea of fresh life; calling something (or someone) "green" means that it's new, innocent, inexperienced. The speaker is saying that the first, fresh growths in Nature, personified as a female entity throughout the poem, are "gold."
Taken literally, "gold" refers to the fact that spring leaves are indeed often a lighter, brighter color than the darker foliage of summer. In fact, new buds on trees are often a very pale green or yellow in color. But "gold" is also symbolic here. Gold is, of course, a precious metal, associated with wealth, beauty, and perhaps purity. Through this metaphor, then, the speaker is saying that the fresh buds of spring are beautiful and valuable. New life, then, is presented as something precious. This connection is underscored by the alliterative hard /g/ sound between "green" and "gold."
At the same time, the language here suggests that things will change, and that something else will follow: the line's focus on "first" green implies that this is a transitional state, and that a different "green"—a different form of life—will come later.
"First green" is given extra significance because of its adjoining stress. It's a spondee, a metrical beat in a line of poetry that has two accented syllables (stressed-stressed). This is only one of two times that a spondee appears in the entire poem, which means its double stress is meant to be noticed—this beautiful "first green" is meant to stand out. The reader is forced to slow down upon reaching this point, and to examine the significance of the words. This ties into the idea of wanting to hold on to something for longer than possible.