As you read this poem, your first hint that the poet is speaking metaphorically is the word "happy" applied to the flower. Flowers are not literally happy or unhappy. They have no feelings, just as they do not "play" (any more than they go about serious business). These metaphors are each built on an implied "as if": It is "as if" the flower had been happily and innocently at play when it was attacked by the frost. It is "as if" the killer frost were an executioner who "beheads" the condemned victim.
The metaphors in this poem make you think of both frost and flowers as if they were human beings, acting out a grim minidrama that stirs your sympathies and raises troubling questions in your mind. (This kind of metaphor, which treats nonhuman objects as if they were human, is an example of personification.)
* First, metaphor has the power to call up impressive visual images. You see with your mind's eye the flower at play, the murderous frost beheading it, the "blonde" assassin passing on nonchalantly. You see (or imagine) the sun proceeding on its course as if nothing significant occurred. Metaphor is one of the poet's chief means of living up to the ideal that "a poem does not talk about ideas; it enacts them" John Ciardi).
* Second, metaphor has the power to stir feelings. You are likely to shudder at the quick devastation of the helpless, hapless flower. You should feel at least a twinge of alarm at seeing it destroyed. The ability of metaphor to engage our emotions makes for a key difference between poetic language on one hand and scientific language or other kinds of emotionally neutral language on the other.