what is the function of {% include filename %} and {{ content }}in this code?i do not find such a syntax :     <!doctype html>
    <html>`enter code here`
    {% include head.html %}
    <body>
        <div class="container jumbotron">
            <h1 class="text-center"><i class="fa fa-paper-plane float shadow"></i> Flyaway<a href="">.css</a></h1>
            <p class="lead text-right">~ created by <small><a href="http://takentech.com/about.html"><i class="glyphicon glyphicon-fire"></i> 進擊的燊</a></small> ~</p>
            <ul class="list-inline text-center">
                <li>
                    <iframe class="github-btn" src="https://ghbtns.com/github-btn.html?user=lushen&amp;repo=flyaway&amp;type=watch&amp;count=true" allowtransparency="true" frameborder="0" scrolling="0" width="100px" height="20px"></iframe>
                </li>
                <li>
                    <iframe class="github-btn" src="https://ghbtns.com/github-btn.html?user=lushen&amp;repo=flyaway&amp;type=fork&amp;count=true" allowtransparency="true" frameborder="0" scrolling="0" width="102px" height="20px"></iframe>
                </li>
            </ul>
            {{ content }}
            {% include footer.html %}
        </div>
        <!-- jQuery (necessary for Bootstrap's JavaScript plugins) -->
        <script src="http://apps.bdimg.com/libs/jquery/2.0.0/jquery.min.js"></script>
        <!-- Include all compiled plugins (below), or include individual files as needed -->
        <script src="http://apps.bdimg.com/libs/bootstrap/3.3.0/js/bootstrap.min.js"></script>
        <script type="text/javascript" src="js/back-to-top.js"></script>
        <script type="text/javascript" src="js/index.js"></script>
        <script type="text/javascript" src="js/prefixfree.js"></script>
        <script type="text/javascript" src="js/shake.js"></script>
        <script type="text/javascript" src="js/flyaway.js"></script>
    </body>   
    </html>
This is a opensource on github. I just want to understand this code.There 
are head.html and footer.html files ,but when i open the default.html file on my computer,the head and footer file do not load on the web page.But on the other hand , when i use the 'check' button on the chrome to see the source code of http://takentech.com/flyaway/ , the head and footer are actually loaded. The same time , i search the internet to find what {{}} and {% include filename%} really are. there is no such a syntax.
finally.Thanks for all the answers.