What is indices meaning in kotlin

Question

I want to know actually how .indices works and what is the main difference is between this two for loops.

for (arg in args)
    println(arg)

or

for (i in args.indices)
    println(args[i])

And what is use of withIndex() function

for ((index, value) in array.withIndex()) {
    println("the element at $index is $value")
}