A JavaScript program to capitalize the first letter of each word in a given string - I don't understand this line of code

I'm new to JS.
I've written a program that capitalizes the first letter of each word in a given string. I also found another piece of code on w3resource.com that does the same, but I don't quite understand how it works.
Here's my code:

<!-- begin snippet: js hide: false console: true babel: false -->

<!-- language: lang-js -->

function capFirstLetters(str) {
  const words = str.split(" ");
  for (let i = 0; i < words.length; i++) {
    const letters = words[i].split("");
    letters[0] = letters[0].toUpperCase();
    words[i] = letters.join("");
  }
  return words.join(" ");
}

console.log(capFirstLetters("i wrote a program that capitalizes the first letter of each word in a string."));

<!-- end snippet -->

Here's the code from w3resource.com:

<!-- begin snippet: js hide: false console: true babel: false -->

<!-- language: lang-js -->

function capital_letter(str) {
  str = str.split(" ");
  for (var i = 0, x = str.length; i < x; i++) {
    str[i] = str[i][0].toUpperCase() + str[i].substr(1);
  }
  return str.join(" ");
}
console.log(capital_letter("i wrote a program that capitalizes the first letter of each word in a string."));

<!-- end snippet -->

What I don't understand is the line of code inside the for loop. Can you work with the letters of a string as elements of an array without first creating an array from the string?

See the other answers for the explanation of the split.

I answer your other question: Yes you can access the string as an array of letters.

However since a string is immutable (cannot be changed in place) we need to copy it anyway. Here I use [...spread] to make the array but I index the string directly

<!-- begin snippet: js hide: false console: true babel: false -->

<!-- language: lang-js -->

const capFirstLetters = str => [...str]
  .map((letter,i) => (i===0 || ".?! ".includes(str[i-1])) ? letter.toUpperCase() : letter)
  .join("");

console.log(capFirstLetters("i wrote a program that capitalizes the first letter of each word in a string."));

<!-- end snippet -->

The split function, as described in w3school documentation, creates an array from a string. The following line creates an array variable called str, from the string variable called str, passed into the function - capital_letter(str)

str = str.split(" ");

So in a console you could do this:

str="i am a string";

Which returns:

'i am a string'

and:

str = str.split(" ");

Which returns:

(4) ['i', 'am', 'a', 'string']

finally:

str[3];

Which returns:

'string'