difference between dot notation and bracket notation in javascript

I am trying to understand the difference between .Notation and [] notation. In my problem below when I use if (object[key] === true) I get the correct answer. When I use if (object.key === true) it does not work. Could someone explain why it's different.

var myObj = {
    one: false,
    two: false,
    three: false,
    four: true,
    five: false,
    six: false
};
var myFunc = function (object) {
    for (var key in object) {
        if (object[key] === true) {
            return "There is a true value in this object";
        } else {

        }
    }
    return "Sorry, there are no true values in this object";
};