`-` is the whitespace controll character. It will strip all whitespace until the first character. This means you can indent your Twig code nicely, without having a weird indentation after rendering the HTML code.

In short:

````
<div>
    {% block content %}{% endblock %}
</div>
----
{% block content -%}
    <h1>Hello</h1>
{%- endblock %}
````
Becomes:
````html
<div>
    <h1>Hello</h1>
</div>
````

While
````
...
------
{% block content %}
    <h1>Hello</h1>
{% endblock %}
````
Becomes:
````html
<div>
        <h1>Hello</h1>
</div>
````