example of what a callback is

callback_example.js

function loadTweets() {
  // first we start loading the tweets with ajax and set the callback to tweetsLoaded
  $.get("/tweets", tweetsLoaded)
  
  // then, we show a message stating that we're loading the tweets
  $("#status").text("loading tweets")
}

function tweetsLoaded(tweets) {
  
  // this function will add the tweets in the variable tweets to the screen
  $("#status").text("tweets loaded")
}

// we execute the function loadTweets when the button is clicked. This is also a callback
$("button").click(loadTweets)

// a timeline of what will happen
// 1. user clicks button
// 2. ajax starts loading the tweets
// 3. status is shown as loading tweets
// 4. tweetsLoaded function is executed
// 4.1. tweetsLoaded function is supposed to add the tweets to the page
// 4.2. status is updated to tweets loaded