He sat in the dark of his bunk and touched a face that no longer felt like his own. A finger sank into a nickel-size depression at the corner of his left eye. He traced the fracture line to his nose, then across to where long scars gathered in the hollow of his cheek. They’d healed white, but prison stitches weren’t the greatest. If time inside had taught him one thing, though, it’s what really mattered in life.
what does the highlighted mean?
It means that the scars are white (imagine an old scar, so it is healed but you can still see it, it is normally a different colour to the rest of the skin, and in this description the scars are white). The sentence also says that, although the scars have healed, they are not the best because they are done in a prison - this implies maybe they are a bit messy, a bit too obvious, because they were not done by top quality doctors to be as small and neat and unnoticeable as possible.
I hope that makes sense :)
Annabel is spot on, I agree. The only thing I would add is maybe this is also an insight into the character's personality. Prison has healed him somewhat, but there is a lot of mess underneath.agree
And the stitches might even have been done by himself or more likely another inmate.