What is .indices meaning in kotlin?

These are just different ways to iterate over an array, depending on what you need access to within the body of the for loop: the current element (first case), the current index (second case), or both (third case).

If you're wondering about how they work under the hood, you can just jump into the code of the Kotlin runtime (Ctrl+B in IntelliJ), and find out.

For indices specifically, this is pretty simple, it's implemented as an extension property that returns an IntRange which a for loop can then iterate over:

/**
 * Returns the range of valid indices for the array.
 */
public val <T> Array<out T>.indices: IntRange
    get() = IntRange(0, lastIndex)

As mentioned in the documentation indices is the index value from the list.

Returns an IntRange of the valid indices for this collection.

For more details refer to the documentation