What is a line by line explanation of the poem "The Sign-Post" by Edward Thomas?

This poem is about how a person's perceptions change over time. It is about the choices we make and in the end, how one should appreciate life. 

The opening four lines illustrate a wintry setting. The "dim sea glints chill." This suggests that the light ("glints") is dim and cold. The sun is "shy" which indicates it is hiding behind clouds. It is "white" and with those clouds, the sky might be overcast. The grass is covered in frost. It is a cold, maybe even melancholy scene. 

The traveler, perhaps smoking a pipe, comes upon a "finger-post." This is simply a horizontal sign that points in a certain direction, perhaps towards a town. However, this idea of the "finger-post" or "sign-post" is metaphoric. It has to do with choices in life.

He reads the sign and wonders which way to go. He hears a few voices but one interpretation is that these are his own voices speaking to him from different times in his life. One voice notes that at twenty years old, he would have known which way to go immediately. However, at that age, he was so pessimistic that he wished he'd never been born. 

In the next section, the hazel shrub loses a leaf of gold. This image shows the passage of time. He wonders what choice he would make at sixty years old. Another voice tells him "You shall see" and they laugh together. The more mature voice explains that no matter what may happen, every life ends in death. "A mouthful of earth to remedy all / Regrets and wishes shall freely be given." Reminding himself of his own death, he tries to use this to find a way to appreciate the time he has on earth. 

If there is some flaw in heaven, he (speaking to himself) will wish to be back on earth no matter what the weather or the circumstances are. This more mature voice criticizes the speaker's pessimistic view of things when he was twenty. He is saying that even if he goes to heaven, he will want to be back on earth. Thus, the poem ends with the speaker admonishing his younger, pessimistic self to appreciate all aspects of life and to embrace and appreciate even having the free will to choose his own paths in life: 

To see what day or night can be,

The sun and the frost, the land and the sea,

Summer, Autumn, Winter, Spring,—

With a poor man of any sort, down to a king,

Standing upright out in the air

Wondering where he shall journey, O where?”