A metaphor is defined as a "comparison which imaginatively identifies one thing with another." Frost doesn't specifically use the phrase "fork in the road" in his poem "The Road Not Taken," but he could have been thinking of the tines of a fork when he wrote the poem.
Particularly if the tines of a fork have been bent, they don't go in the same direction. If you try to use this bent fork to eat, you must make a decision about which side of the fork to use - which part of the bent collection of tines will work best to stab a bite of food.
In the same way, there are times in life when a person faces a moment of decision because there are multiple possibilities, but only one can be followed. At that point, the person is required to study all the possible courses of action and then make a choice, based on the best information available.
The metaphor of the "fork in the road" compares making life's decisions to following the tines of a fork.