We've got a widow. And she's lamenting. Oh, and it's springtime. Consider the scene set.
Let's start with those first three words: "The Widow's Lament." By saying "The Widow" the title already gives us a sense of isolation. It sounds like she's the only widow in the world! After all, if there were many, she would just be "A Widow." The word "The" is single, and imparts a sort of loneliness to her character. Plus, calling someone a widow has a way of defining them entirely by their lost husband. When we think of them as "widow," they are defined by that loss.
And that's precisely the point, isn't it. In giving us this title, the poem has prepared us for the way that our speaker's loss of her husband has come to overwhelm other aspects of her life (like her connection to the natural world, and to her son). The word "Lament" adds extra force to the sense of loss and sadness that was already introduced in the idea of a widow.
Finally, in mentioning spring, the poem has prepared us, without us really knowing it yet, for the contrast between the renewal, brightness, and life of spring and the sadness and irrevocability of death. The widow's lament would be sad in the winter of course. But because it's spring, it's all the sadder, because the outer world won't mimic her inner state.