indexer index initialization

I have this piece of coding related to the undexer:

/ Use an indexer to create a fail-soft array.
using System;
class FailSoftArray {
int[] a; // reference to underlying array
public int Length; // Length is public
public bool ErrFlag; // indicates outcome of last operation
// Construct array given its size.
public FailSoftArray(int size) {
a = new int[size];
Length = size;
}
// This is the indexer for FailSoftArray.
public int this[int index] {
// This is the get accessor.
get {
if(ok(index)) {
ErrFlag = false;
return a[index];
} else {
ErrFlag = true;
return 0;
}
}
// This is the set accessor.
set {
if(ok(index)) {
a[index] = value;
ErrFlag = false;
}
else ErrFlag = true;
}
}
// Return true if index is within bounds.
private bool ok(int index) { //>>>>>>   DOUBT >>*
if(index >= 0 & index < Length) return true;
return false;
}
}
// Demonstrate the fail-soft array.
class FSDemo {
static void Main() {
FailSoftArray fs = new FailSoftArray(5);
int x;
// Show quiet failures.
Console.WriteLine("Fail quietly.");
for(int i=0; i < (fs.Length * 2); i++)
fs[i] = i*10;
for(int i=0; i < (fs.Length * 2); i++) {
x = fs[i];
if(x != -1) Console.Write(x + " ");
}
Console.WriteLine();
// Now, display failures.
Console.WriteLine("\nFail with error reports.");
for(int i=0; i < (fs.Length * 2); i++) {
fs[i] = i*10;
if(fs.ErrFlag)
Console.WriteLine("fs[" + i + "] out-of-bounds");
}
for(int i=0; i < (fs.Length * 2); i++) {
x = fs[i];
if(!fs.ErrFlag) Console.Write(x + " ");
else
Console.WriteLine("fs[" + i + "] out-of-bounds");
}
}
}

The output from the program is shown here:
Fail quietly.
0 10 20 30 40 0 0 0 0 0
Fail with error reports.
fs[5] out-of-bounds
fs[6] out-of-bounds
fs[7] out-of-bounds
fs[8] out-of-bounds
fs[9] out-of-bounds
0 10 20 30 40 fs[5] out-of-bounds
fs[6] out-of-bounds
fs[7] out-of-bounds
fs[8] out-of-bounds
fs[9] out-of-bounds
The indexer prevents the array boundaries from being overrun. Let’s look closely at
each part of the indexer. It begins with this line:
public int this[int index] {

*question] please see the arrowed "doubt" line

 How does the following  part gets the value initialised with the value of index?

private bool ok(int index) {