javascript - how to use substring function and what does this mean?

so i'm currently trying to figure out what the code below will output. I'm confused by the substr function so if you could explain that it would be amazing. Thanks

    function getAttackString() {
    var foo = "d32329b34";
    var bar = "x38h309hj";
    return "The code is: "+(foo.substr(3,foo.length-6))+(bar.substr(2));
    }
    
    console.log(getAttackString());