It might be best to begin with a famous poem, The Road Not Taken." The entire poem is a metaphor for the decision we make in life which, while they may not seem all that important in the moment, may determine the rest of our lives ... and they are decisions that we can never go back and "remake."
In the beginning of the poem we are presented with two roards that diverge in a yellow wood. I'm not sure why the woods were "yellow," but it has always suggested Fall to me and that the decider is not a young person... which may make the decision all the more important because there is less time to alter the ramifications of each decision. And the location of the decision (he's not deciding between options on a superhighway) is a metaphor for the undertainty of life (think Young Goodman Brown or The Devil and Tom Walker to point out just two stories where the woods function in this way). With no real clues about which way to go (they are worn about the same --- no help there), the traveller makes a physical/metaphorical decision --- to take one path, live it out, and later tell the story how this decision has made all the difference ... both in the trip and in his/her life.
Another example is in his poem "Birches." In this poem he uses climbing birches and returning back to earth as a metaphor for the imaginative, risk taking part of life, before the "facts" of life take over for each of us:
WHEN I see birches bend to left and right Across the line of straighter darker trees,
I like to think some boy's been swinging them. But swinging doesn't bend them down to stay. Ice-storms do that. Often you must have seen them 5
Loaded with ice a sunny winter morning
After a rain. They click upon themselves
As the breeze rises, and turn many-colored
As the stir cracks and crazes their enamel.
Soon the sun's warmth makes them shed crystal shells 10
Shattering and avalanching on the snow-crust— Such heaps of broken glass to sweep away You'd think the inner dome of heaven had fallen. They are dragged to the withered bracken by the load,
And they seem not to break; though once they are bowed 15
So low for long, they never right themselves: You may see their trunks arching in the woods Years afterwards, trailing their leaves on the ground
Like girls on hands and knees that throw their hair
Before them over their heads to dry in the sun. But I was going to say when Truth broke in With all her matter-of-fact about the ice-storm 20
In Frost's own words:
," Poetry begins in trivial metaphors, pretty metaphors, 'grace metaphors,' and goes on to the profoundest thinking that we have. Poetry provides the one permissible way of saying one thing and meaning another. People say, 'Why don't you say what you mean?' We never do that, do we, being all of us too much poets. We like to talk in parables and in hints and in indirections - whether from diffidence or from some other instinct". ... Excerpt from an essay entitled "Education by Poetry" by Robert Frost.
Most of Robert Frost's poetry contains or is built upon a central metaphor. While these metaphors may all be different, there are a few things he keeps central throughout his poetry: a search for something (for many writers in History it was a search for truth, sense of self, or God), and turning to nature to find it (reminiscent of the Romantic Era).
In arguably his most famous poem "The Road Not Taken" it seems Frost is comparing life or a decision to a split in a road. While the actual meaning of this poem is ambiguous (as much great poetry is) - debates hold that the road either represents life itself, a regretted (or non-regretted) decision, or an opportunity.
"Stopping by Woods" is a poem that is an extended metaphor for slowing down and enjoying the journey of life.
"Fire and Ice" holds metaphors for how the world will end.
Many others are discussed in the links below.