Nature's first green is gold (1)
From the get-go, we can see that our speaker is pretty close with nature. He has paid close attention to the spring and knows that, before trees and plants get to their summer-green color, they are often gold or yellow. This dude clearly knows his green and gold stuff.
Her hardest hue to hold. (2)
Our speaker is so close with nature that he personifies it as… her. She isn't able to hold on to that first gold color for long, which fits with the poem's title, "Nothing Gold Can Stay." By this point, we're pretty sure the poem is talking about the colors of spring, not precious metals.
Her early leaf's a flower; But only so an hour. (3-4)
These lines use metaphor to talk about nature, comparing a leaf to a flower. This, again, is referring to spring blooms, but it's also saying that flowers are not too different from leaves. We're starting to see what the whole "Nothing Gold Can Stay" thing is about—the quick change from spring to summer, from flower to leaf.