Years ago I was on a visit to Wales with my wife, traveling by train. I struck up a conversation with the conductor (yes, that many years ago) and the three of us chatted for ten or fifteen minutes. He was a very Welsh Welshman, with an accent you could cut with a knife and talkative to boot. After he left us, my wife said, "Did you understand a word he said? I didn't."
Some years afterwards we were south of Yellowstone National Park, found a small place for lunch, and I tried to order a bagel. Tried, because the girl at the counter could not understand what I was asking for. Low fat, no butter, trim the ham please was all I was asking for, and in English. Nearly went away hungry but, thankfully, my wife intervened. "How'd you do it?" I asked her. "Simple. Translated from English to American," she said.
Which has what to do with your character voicing? How does this help with getting the right voice for your characters?
Giving characters an individual, recognizable voice is important. It adds realism, gets your readers immersed, and it lends credibility to your writing. Your novel is set in the deep South USA cotton fields of the 1800s? The workers don't sound like Oxford dons.
It's tempting to go overboard, add too much twist and twang, special dialect or accent, misspell words to read as they're spoken, or fill your characters' dialogue with only 'local color'. In the end your characters could end up like my Welsh conductor, or the American bagel girl. Absolutely authentic, perfect fit to the character; and absolutely impossible for your readers to understand.
In my bookshelf I have Iain M. Banks' 1994 science fiction novel 'Feersum Endjinn'. It's a great read, apparently, but in all the years I've had it I have only finished it twice; and then it's been a chore, not a pleasure. Why? Because the main character, Bascute, has a voice that is so twisted, so divergent, so far away from standard English I have to study the words to understand their meaning. The result is that I can't actually read the novel, or enjoy it, the way the author intended. And, worse yet, I've tended away from Mr Banks' works. I'm probably the poorer for it.
If your characters can't be understood by your readers, you've lost the whole point of your writing. Don't drown your characters' voices; a light touch goes a long way. Give them a little flavor, a light dusting; don't drown them in salt.