Can someone explain this string replacing method(SHORT CODE)

Question

The method is replacing "?" with corespondent letter when its clicked(Hangman game)

I cant figure out what offsetBy: location is here for
And replacingCharacters(in: index...index

if word.contains(letter){
let char = (Character(letter))
var location = 0
for c in word{
if c == char {
let index = promptWord.index(promptWord.startIndex, offsetBy: location)
promptWord = promptWord.replacingCharacters(in: index...index, with: String(c))
currentWord.text = promptWord
}
location += 1
}
}

THIS is just a loadlevel method to show for better understanding if its needed:

func loadlevel(){

if let startWordUrl = Bundle.main.url(forResource: "start", withExtension: "txt"){
if let startWord = try? String(contentsOf: startWordUrl){
var allWords = startWord.components(separatedBy: "\n")
word = allWords.randomElement()!

for letter in word {
usedLetter.append(letter)
promptWord.append("?")
}

currentWord.text = promptWord
}

}