In the first stanza of the poem, Hardy tries to recall the woman who has died, and who he loved. He calls her his "lost prize," indicating that he regrets not trying harder to make a relationship with her work. He also describes her as a woman whose "dreams were upbrimming with light," and who had "laughter in her eyes." These images are very positive, and suggest a woman who was happy and hopeful. Light is often symbolic of hope, clarity and joy, and the light imagery used here suggests that the woman was overflowing, or "upbrimming" with these characteristics.
In the second stanza Hardy imagines what this woman's final days may have been like. He again uses light imagery but this time to suggest a passing of light and the coming of darkness. Hardy speculates that her "life-light decline[d]," and here the light symbolizes her life, and the darkness the death that awaited her. Hardy also wonders if the darkness of death took away some of the woman's joy and happiness, and "Disennoble[d]" her soul.
In the third and final stanza of the poem, Hardy says that he now struggles to evoke the image of the woman as she was when she was young, when she was full of light and full of life, and that his memory of her now has been reduced to "but the phantom" of the woman. In other words, the image of the woman has faded in his memory, and is now like the echo of a sound, or the reflection of an image. It is on this rather melancholy note that Hardy ends the poem.