What is html5shiv.js and what it does

Question

<!--[if lt IE 9]>
            <script src="/assets/js/libs/html5shiv.js"></script>
        <![endif]-->

i like to know What is html5shiv.js and what it does ?

if i just include it then my job will be finished and the js file will work automatically?

please guide me how to use html5shiv.js. thanks