a String is a type of data in Javascript, and as with any other type of data, Strings have prototypes and therefore inherit standard methods. One of these methods is toLowerCase(), which you'll often see written as String.prototype.toLowercase(). This method turns any string from any case to just lowercase instead.
Here is a quick example:
let myString = 'HELLO WORLD'
console.log(myString.toLowerCase()) // hello world
This will work on anything which is of string type - so trying to initiate a new String() and use this method also works:
let myString = new String('HELLO!')
console.log(myString.toLowerCase()) // hello!