in JS, what is the itemKey parameter for index(itemKey)? and how am i suppose to know it?

almost every collection in illustrator seems to have the method index(itemKey) that gets an element from the collection with itemKey being 'string, number'

but what is that string and number? how do i find what the "itemKey" of an object is? it's not something i can assign, so when does one use this method without knowing what the itemKey is?