Counting index starts from zero, so for the first letter "H" the index is 0, so in your case charAt(3) for the string is the second "I".
String's characters start at 0.
0 = H
1 = e
2 = l
3 = l
4 = o
So , the answer to your questions is charAt(3) would be the second "l".
The first character in each string starts at zero. So the third index would be the fourth character in that string. In this case the fourth character in "Hello" is "l".
You start counting the index of a string from 0.
For example, for the string "Hello", H will be 0 and o will be 4.
So charAt(3) will be the second "l".