Can someone explain me the logic behind var and let in this situation?

for(let i=0; i<5;i++) {
    setTimeout(() => {
    console.log(i);
  }, 1000);
}

Prints 0 1 2 3 4

for(var i=0; i<5;i++) {
    setTimeout(() => {
    console.log(i);
  }, 1000);
}

Prints 5 5 5 5 5 5