count the frequency of given letters

gistfile1.js

// Write a function charFreq() that takes a string and builds a frequency listing of the characters contained in it. 
// Represent the frequency listing as a Javascript object.
// Try it with something like charFreq("abbabcbdbabdbdbabababcbcbab").
// I got a lot help from stack on this one. I had no idea how to make a object from a string.
// and in doing research for that I found a solution to something similar to this. But I do understand
// how every line of code works and why!


function charFreq(source) {
  var frequency = {};
  for (var i = 0; i < source.length; i++) {
    var character = source.charAt(i);
    if (frequency[character]) {
      frequency[character]++;
    } else {
      frequency[character] = 1;
    }
  }
  return frequency;
};

console.log(charFreq("aabbabcbdbabdbdbabababcbcbab"));

pls can you explain each line of code for better understanding