I'm confused with declaring a let variable , closure, functions and lexical environment in JS

Question

My prof provided me with the following code:

function makeCounter() {
  let count = 0;

  return function() {
    return count++;
  };
}

let counter = makeCounter();

alert( counter() ); // 0
alert( counter() ); // 1
alert( counter() ); // 2

I know that counter() is being called. But I don't see any function named as counter. Could someone shred some lights?