Variable

You mean this block of code, right?:

ul.addEventListener('change', (e) => {
  const checkbox = event.target;
  const checked = checkbox.checked;
  const listItem = checkbox.parentnode.parentNode;
});

This code targets the various individual list items in the unordered list, specifically the one that is 'changed' (its checkbox was clicked and therefore it was toggled or 'changed'). The 'e' in this line of code:

ul.addEventListener('change', (e) => {

is the event object that is always available to the EventListener. One confusing thing is that the same variable (the event object) is referenced in two ways: as 'e' in the first line of code and as 'event' in the second line, but it's the same object (somehow javascript allows it - weird and a bit confusing - but the two ways to reference the object are interchangeable and the code still will work). Anyway, this line:

const checkbox = event.target;

stores the 'clicked' checkbox in the constant 'checkbox' this line:

const checked = checkbox.checked;

stores the current state of that checkbox (either true or false - as in checked or not)

this line:

const listItem = checkbox.parentnode.parentNode;

targets and stores the entire list item element (the parent of the parent of the checkbox that was changed) in the const listItem

The nutshell version is this code enables you to target and manipulate the specific current list item that the user has interacted with.

I hope that helps. Happy coding!