The whole point of the state is to take an existing state, apply an action to it, and produce a new state. It can be written like this:

old state + action = new state

Imagine, if you were performing basic calculations, then you would start writing it like this:

// sum is 0
let sum = 0;

// sum is now 3
sum +=3;

The Redux way of doing things, though, is to change the preceding to:

let sum = 0;
let sumWith3 = sum + 3;
let sumWith6 = sumWith3 + 3; 

We don't mutate anything but rather produce a new state for everything we do. Let's look at different constructs and what it means in practice to not mutate.