It is so very, very useful! It means "yes", sort of, but maybe a fuller translation would be "I agree with what you said." It is just used all the time, since the Korean language lets a lot of things be implied by context. It can mean, "I heard you" or "That's true" or "Well... okay, then." It's good for everything from "I swear it's the truth!" to "uh huh..."
It's especially cool walking into a mom-n-pop store or a traditional market:
me: "Yes, hello."
host: "Yes, hello."
host: "Thank you."
me: "Yes. Goodbye" (the "goodbye" that means "I'm leaving.")
host: "Yes, goodbye." (the "goodbye" that means "I'm staying here.")
It's such a relief, when dealing with a language that is so complex that it has different "goodbyes" depending on who's leaving, and two completely different sets of numbers, and seven layers of word endings depending on the relationship between the speakers, to have something so simple and versatile.
me: "Didn't you do your homework?"
student: "Yes." (I didn't.)
...which takes some getting used to, but, if you think about it, makes sense.
I find myself using "neh" all the time, even with English speakers who live here. It will take a while to get over it; I spent some time in another South, dangling off the eastern side of another continent, and I still love saying "y'all".
I just got back from Costco, where the checkout lines are interminable. The Korean man in front of me had a moderately full cart. He saw me with my lone item, a bag of frozen ravioli, and asked (in English): "Just one?"
I said "neh". And he motioned for me to go in front of him.
In Korean, the word for "information" is "안내", which may or may not stand for "no yes". Maybe I'll stop at an 안내 booth and ask, "Doesn't "안내" come from 'no yes'?"
The answer will probably be "neh". (It doesn't.)
Just say "neh" to language.