function checkHoisting() {
  console.log(foo); // undefined
  var foo = "Foo";
  console.log(foo); // Foo
}

checkHoisting();

function run() {
  var foo = "Foo";
  let bar = "Bar";

  console.log(foo, bar); // Foo Bar

  {
    var moo = "Mooo"
    let baz = "Bazz";
    console.log(moo, baz); // Mooo Bazz
  }

  console.log(moo); // Mooo
  console.log(baz); // ReferenceError
}

run();

The reason why let keyword was introduced to the language was function scope is confusing and was one of the main sources of bugs in JavaScript.

Take a look at this example from another Stack Overflow question:

var funcs = [];
// let's create 3 functions
for (var i = 0; i < 3; i++) {
  // and store them in funcs
  funcs[i] = function() {
    // each should log its value.
    console.log("My value: " + i);
  };
}
for (var j = 0; j < 3; j++) {
  // and now let's run each one to see
  funcs[j]();
}

My value: 3 was output to console each time funcs[j](); was invoked since anonymous functions were bound to the same variable.

People had to create immediately invoked functions to capture correct values from the loops but that was also hairy.

Hoisting

Variables declared with var keyword are hoisted and initialized which means they are accessible in their enclosing scope even before they are declared, however their value is undefined before the declaration statement is reached:

function checkHoisting() {
  console.log(foo); // undefined
  var foo = "Foo";
  console.log(foo); // Foo
}

checkHoisting();

let variables are hoisted but not initialized until their definition is evaluated. Accessing them before the initialization results in a ReferenceError. The variable is said to be in the temporal dead zone from the start of the block until the declaration statement is processed.

function checkHoisting() {
  console.log(foo); // ReferenceError
  let foo = "Foo";
  console.log(foo); // Foo
}

checkHoisting();