When we're dealing with a poem based in common measure, it's typical for the poet to use the poem's title as the first line. Dickinson does the same thing with another one of her poems also written in common measure, "Because I could not stop for Death." In fact, she does it for most of her poems, as did many of her contemporaries and predecessors.
So it's a fairly typical practice that serves to introduce the poem in a straightforward kind of way. Maybe it's even similar to the way you'd introduce a person, by first giving their name and then fleshing out their details in conversation. But the title itself, "There's a certain Slant of light," provides more than just the poem's name. It also gives us a taste of the kind of informal diction and mysterious context that the speaker uses throughout the poem.
Notice we have a contraction "There's" instead of a fully spelled out "There is." Likewise, we have some ambiguity in the word "certain" that suggests the speaker isn't looking to be overly descriptive. So in the title alone we get a sense of the sort of angle the speaker is looking to use in this poem. She's keeping things relatively informal, casual, and ambiguous in order to better contextualize the mysteriousness of that "Slant of light."