Lines 1-6 introduce the poem's speaker, whom the title identifies as a "Widow" grieving in "Springtime." (A "Lament" is an expression of grief or sorrow.) The widow's gender, age, personal circumstances, etc. differ from those of William Carlos Williams, so this is a dramatic monologue: a poem whose speaker is a character separate from the poet.
These opening lines don't say who or what the widow is lamenting; that information is withheld until lines 7-8. Instead, they focus on her experience of "Sorrow." She begins with the statement "Sorrow is my own yard," which suggests both that her yard makes her sad and that, metaphorically, sadness is like her own yard. In other words, it's a state so familiar that it seems to belong to her, like a piece of private property.
She goes on to describe her literal yard in more detail. Metaphorically, the bright, quickly-growing "new grass" spreads like fire, or "flames," over the yard. It's a familiar sight—"it has flamed / often before"—but the widow's response has changed "this year." This year, she experiences the sight, and perhaps the feel, of the grass growing around her as a "cold fire / that closes round."
The oxymoronic phrase "cold fire" could suggest that the grass looks like flames but feels cold to the touch. However, it seems to have more to do with the widow's emotions. Perhaps her emotional response to the grass—or the springtime in general—feels like a mix of passion and detachment, or pain and numbness.
Here and throughout the poem, the free verse is chopped into short, heavily enjambed lines, which suggest that the widow speaks with a halting or short-winded rhythm. At the same time, these first six lines form a run-on sentence. Perhaps, then, this is a speaker who has a lot to say, but who has some difficulty getting the words out, due to strong emotion or other factors.