I was reading something about how arrays access their elements in a C++ book and I don't understand.

Because the arrays, in C/C++ programming languages, are 0-based: the 'first item' is the 'item 0', the 'second item' is 'item 1' and so on:

int a[3];
a[0] = 15; // assign 15 to the first item of the array, that is the item 0.
a[1] = 12; // assign 12 to the second item of the array, that is the item 1.
a[2] = 42; // assign 42 to the third item of the array, that is item 2.
// note there is NO a[3] (because it would be the fourth item)

The following program,

#include <iostream>
using namespace std;

int main()
{

  int a[3];
  a[0] = 15; 
  a[1] = 12; 
  a[2] = 42; 
  
  cout << "the address of a is: " << a << endl;

  for ( int n = 0; n < 3; ++n)
  {
    cout << "the address of the " << (n+1) << "-th item, that is &a[" << n << "] is: " << &a[n] << endl;
  }
}

on my Linux box, outputs

the address of a is: 0x7ffd1275d54c                                                                                                                        
the address of the 1-th item, that is &a[0] is: 0x7ffd1275d54c                                                                                             
the address of the 2-th item, that is &a[1] is: 0x7ffd1275d550                                                                                             
the address of the 3-th item, that is &a[2] is: 0x7ffd1275d554    

Quote:

Im sorry but i still don't see why

Think about it: you were born on October 23rd 2000, so your first birthday was October 23rd 2001, your second on October 23rd 2002, and so on. The year is indexed from 2000, the birthday numbers run 1st, 2nd, 3rd, and so on.

This called zero-based and one-based indexing. "Human values" such as age run with a "one-based index" because we never say "my baby is zero years old", or "he is 0 months and five days", computers run with "zero-based" indexes.

So the first element (human term) of an array is at index zero, the second is at index 1, and so on.

string[] human = new string[] {"1st", "2nd", "3rd"};
for (int i = 0; i < human.Length; i++)
   {
   Console.WriteLine($"The {human[i]} element is at index {i}");
   }

So the element at index N will be the "N + 1"th element.

Does that make sense?