let = Lets set a variable that can be mutated/altered later
let age = 23;
age = 24;
No console error
const = set constant variable that should not be mutated/altered later
const birthYear = 1980;
birthYear = 1981;
var = should not be used in modern (2016+) Java script.
var; was the precursor to let and const , found only in pre 2016 Js code.
A declaration of let declares a set of Variables that can be altered.
A declaration of const declares a set of Variables that should not be altered.
var is pre 2016 JS update and was used before the introduction of let and const