If you turn the feature on, you'll get more detailed voice guidance. It might even be useful for folks with perfect vision traveling somewhere they've never been before.
For example, instead of just getting alerts for the next turn you need to make, the improved guidance can tell you how much farther you need to walk before you make the turn, and if you're about to cross a busy intersection. Google also gives frequent updates that you're on the right route, according to a blog post from the company. And it will tell you if you've taken a wrong turn.
It's available in English in the U.S. and in Japanese in Japan.
I couldn't find the option on Google Maps on my iPhone yet, but it usually takes a few days for these features to roll out to everyone. You can check by opening Google Maps on your phone, going to navigation, scrolling down to the section under "walking options" and turning on "detailed voice guidance."