The item() method is used to return a list item from where it’s position is on the index. Therefor it uses the index property which is where the position of the wanted element is placed in. Since this can be used for both Class Lists and arrays you can use brackets and parentheses to place the index value in. It also uses the zero-index property to put the items in order. On the example below you can see that it picks the 2nd, in zero-index, classList to put as content for the heading.

//Lets say we had a heading with the class "Pick what's relavent"
const head = document.querySelector("h1");
const classes = head.classList;
const item = classes.item(2);
head.textContent = item;
//The HTML output would be a heading saying "relavent"