Hi I am totally lost on this code challenge, someone please go step by step with detailed explanations

var utilities = require("./utilities");

// Create mailValues object
var mailValues = {};

// Add 'first_name'  to mailValues object
// mailValues = { first_name: "Janet" }
mailValues.first_name = "Janet";

// Content has the 'first_name' surrounded by '%' to differentiate items to be replaced
var emailTemplate = "Hi %first_name%! Thanks for completing this code challenge :)";

// Pass both of these items into the utility
var mergedContent = utilities.merge(emailTemplate, mailValues);

//mergedContent === "Hi Janet! Thanks for completing this code challenge :)";

function merge(content, values) {

  // Loop through all values and replace the place holder with the associated content
  // values == { first_name : "Janet" }
  // So the loop below would do:
  // content = content.replace("%first_name%", "Janet");
  for (var key in values) {
    content = content.replace("%"+key+"%", values[key]);
  }

  return content;
}

module.exports.merge = merge;