You know how you can pass all sorts of things as arguments into a function?

function iEat(thing) {
  console.log("I eat " + thing);
}
iEat(5);
iEat("lots of things");
iEat(7.9840);
iEat(["pork", "beans"]);

NOW! Imagine we give a function a function as an argument!

iEat(thing, aFunction) {
  console.log("I eat " + thing);
  aFunction(thing);
}

Now, you can take the iEat(thing, aFunction) function, and make the second argument a function!

function myFunction(myFunctionArgument) {
  console.log("I am " + myFunctionArgument);
}

iEat(thing, aFunction) {
  console.log("I eat " + thing);
  aFunction(thing);
}

So, step by step:

iEat("Bananas", myFunction);
// ------------------------------
// You give iEat the word Bananas and the previous function
// we created, myFunction.
// Below, I'm going to put the individual arguments into 
// the function to try and make it easier to understand
//-------------------------------
iEat("Bananas", myFunction) {
  console.log("I eat " + "Bananas");
  myFunction(thing);
}

So when your function iEat makes a call to another function, aFunction this is considered a callback!