The speaker says he has been acquainted with the night. "Acquainted" is a funny word to use – it means, basically, that he has met, or has some knowledge of, the night. It is a formal, distant, and neutral way to say something. You'd say you were acquainted with someone if you had met them, but weren't friends with them.
So, the speaker may know the night, but he's not saying that he likes it very much. He's not saying that he dislikes it, either. Just that he's acquainted.
Then, not only does the speaker use "acquainted," a strange word to talk about knowing something, he says it in a strange way – he "has been one," instead of he "has been." It's possible that this is written this way to keep the steady rhythm of the poem (see "Form and Meter"), but the idea of having been "one" works with the lonely tone of the poem.
The word "one" also suggests "one of many." There could be others who are acquainted with the night.
Pay attention to this line – part of it is in the title, and it is both the first and last line of the poem. This repetition makes this line very important to the meaning and effect of the poem.
I have walked out in rain – and back in rain.
The speaker has walked out and back – to and from home, or wherever he was – in the rain, and as we know from the first line, at night. This doesn't sound like very much fun.
Instead of just saying he's walked out and back in rain, the speaker repeats the phrase "in rain" twice. This repetition makes us really feel how miserable walking around in the rain at night is.
I have outwalked the furthest city light.
Here, the speaker tells us that he's walked past the furthest city light. We're not sure whether he's walking outside of the city and into the country, or into a part of the city where there are no lights. But wherever he's walking, it's dark.
This line is creepy. Though the use of the past tense throughout the poem ("I have walked") makes us think that that all of these walks may have happened at different times, we start to envision one rainy, dark night.
We are so deep in the depths of this lonely night that there's nothing, not even a glimpse of a city light, to guide our way.