for loop and setTimeout with VAR

The Code is:

for (var i = 0; i < 5; i++) {
  setTimeout(() => {
    console.log('The index of this number is: ' + i);
  }, 0);
}

And the output is 5 times the line

The index of this number is: 5

for loop and setTimeout with LET

The Code is:

for (let i = 0; i < 5; i++) {
  setTimeout(() => {
    console.log('The index of this number is: ' + i);
  }, 0);
}

The output is:

The index of this number is: 1
The index of this number is: 2
The index of this number is: 3
The index of this number is: 4
The index of this number is: 5

Why the output is different ? Why does this work like this ?