"Nothing Gold Can Stay" relies on imagery of the natural world, like leaves, flowers, and sunrises, to make meaning. But the speaker doesn't just describe nature directly. He uses figurative language, like metaphor and personification, to talk about it. That means that what he says about nature can be broadened to figuratively say something about humanity.
Line 1: Here Frost starts off talking about nature, using a metaphor: "green is gold." This takes the idea that green is the color we normally associate with nature and spring and twists it to show that, at the beginning of spring, nature is actually more gold than green. Note that this line also uses alliteration, repeating the G in "green" and "gold," which only adds to the connection between the two colors.
Line 2: This line uses personification to talk about nature. It refers to nature as a "her" and says that she has a hard time holding on to the color gold.
Line 3: This metaphor, comparing a leaf to a flower, blurs the line between the two.
Line 5: This line expects you to remember the metaphor from line 3, which compared a leaf to a flower. If you translate the line to "flower subsides to leaf," it makes a little bit more sense. The speaker's use of the word leaf twice keeps our minds working and adds more alliteration to the poem.
Line 6: This line uses a biblical allusion to refer to nature: the Garden of Eden. In the Bible, the Garden of Eden is a perfect natural paradise.
Line 7: Here we're shown something about nature that we can see even if we're in the middle of a city—the sunrise. Note, though, that the line doesn't talk about the sun coming up, but dawn going down to day. Weird, right?