Suppose you have an array, like this:
const colors =['red','yellow','green','blue']
In this case the array has 4 items.
You know you can get the first item using colors, the second using colors and so on.
To get the last item without knowing beforehand how many items it contains, you can use the length property to determine it, and since the array count starts at 0, you can pick the last item by referencing the <array>.length - 1 item.
Here's a simple example:
const lastItem = colors[colors.length -1]