Use Bracket Notation to Find the Last Character in a String

https://www.freecodecamp.org/challenges/use-bracket-notation-to-find-the-last-character-in-a-string

Here is the example given:

var firstName = "Ada";
var lastLetterOfFirstName = firstName[firstName.length - 1];

Why does the code have firsstName[firstName? Why can’t you just start with the bracket so the code would read [firstName.length - 1];

I am a little confused as why it is necessary/ what the purpose is?

Thank you!