Let & Var different output

Question

Here are the two code snippets, I am using "var" keyword in one and "let" in another. why the outcomes are so different.

for (let i = 0; i < 5; i++) {
  setTimeout(function() {
    console.log(i);
  }, 100);
}

output: 0 1 2 3 4

for (var i = 0; i < 5; i++) {
  setTimeout(function() {
    console.log(i);
  }, 100);
}

Output: 5 5 5 5 5