means that,where are the variable declaration is working simply speaking, you can get the value of this variable in this scope, but outside of this scope you can not access them.

// function scope

function here() {
  var a = 5;
}
here();
console.log(a); // you got a is not defined

// block scope

/*
a block scope is { } these curly brackets
*/

if (true) {
  let a = 5;
}
console.log(a); // you got a is not defined

Simply speaking, when declaring variable the declaration will be hoisted, and assing value will not Hoist, you can think it is like to move the declaration to the top

the keyword Var, when hoisting happen, it will initalize the value as undefined first.

And declaration key: let and const They do hoisting, too. But they are not going to initialize the varaible as undefined, the variables go to Temporal Dead Zone first. Until code goes to the declaration line, it will be assign value, then you can access the variable

// 範例：

console.log(a); // you got undefined
var a = 6;

console.log(b); // you got ReferenceError: b is not defined (in browser)
// ReferenceError: Cannot access 'a' before initialization。 （in node）
let b = 6;

// hoist to declaration, the Temporal Dead Zone

console.log(b); //

let b;
/*
1. let b hoisting to the top: in TDZ
2. pregram goes to console.log(b) : Cannot access 'b'
3.pregram goes to let b: assign undefined to b , and leave TDZ
*/