javascript - Iterate through object properties

var obj = {
    name: "Simon",

    age: "20",
    clothing: {
        style: "simple",
        hipster: false
    }
}

for(var propt in obj){
    console.log(propt + ': ' + obj[propt]);
}

How does the variable propt represent the properties of the object? It's not a built-in method or property. Why does it come up with every property in the object?