The poem opens with the word "We." Already, the reader knows that the speaker is making a universal statement about humanity: that everyone "grow[s]" grow accustomed to the Dark / When Light is put away." That is, everyone gets used to darkness when the light is gone.
Nowadays, people are more used to turning a light off than putting it away—so in line 2, the reader might imagine the neighbor holding, not an electric porch light, but an old-fashioned handheld kerosene lamp (like this). The speaker imagines this neighbor holding this lamp up to say goodnight, and then leaving—taking the light of the lantern with her.
Think about how your eyes slowly adjust when you're standing in a dark room—from not being able to see at all, to slowly making out shapes. That's what the speaker is describing, but applying it to human experience in general, not just eyesight.
In literature, light usually stands for knowledge, warmth, and understanding, while darkness often represents opposite qualities—fear, sadness, the unknown, and so on. This symbolism is important to making sense of the poem.
It's not so easy to make one's way through that kind of darkness! Dickinson's characteristic dashes at the end of lines 1, 2, and 4 make the poem feel a little tentative, as though it is fumbling about in the dark to find its footing.
It's also worth noting how the poem offers no information about the neighbor's identity—nor, in fact, that of the person she bids farewell. This lack of specifics gives the poem a mysterious atmosphere, and helps readers understand that this opening scene isn't meant to be taken literally. Instead, the speaker is using an extended metaphor that compares human resilience through tough times to the way that people's eyes adjust to darkness.