let is block scoped

A block is a chunk of code bounded by {}. A block lives in curly braces. Anything within curly braces is a block.

So a variable declared in a block with let  is only available for use within that block. Let me explain this with an example:

let greeting = "say Hi";
let times = 4;

if (times > 3) {
    let hello = "say Hello instead";
    console.log(hello);// "say Hello instead"
}
console.log(hello) // hello is not defined

We see that using hello outside its block (the curly braces where it was defined) returns an error. This is because let variables are block scoped.