What are the numbers next to links to style sheets in markup?

I'm curious about these numbers: 100, 99, 98, 97 . I haven't come across something like this yet, is it TWIG or is it HTML that I have not come across yet?

TWIGCopy

{% block stylesheets %}
    {% do assets.addCss('http://yui.yahooapis.com/pure/0.6.0/pure-min.css', 100) %}
    {% do assets.addCss('https://maxcdn.bootstrapcdn.com/font-awesome/4.5.0/css/font-awesome.min.css', 99) %}
    {% do assets.addCss('theme://css/style.css', 98) %}
    {% do assets.addCss('theme://css/custom.css', 97) %}
{% endblock %}
{{ assets.css() }}

Hi, it's priority of loading. The higher, the first to be added to the HTML generated. Check this example.