To comment out one or more lines of code (or part of a line) use the the {#   #} syntax.

For example:

{# This will be a comment #}

<p>You can also comment out {# part of a line #}.</p>

{# This will be a

Comments aren't just useful for writing notes about your code. You can also cause blocks of code to not be executed. Any Twig code that is between comment tags will not be exectuted or outputted. 

{# The following code will not be executed and nothing will be outputted
{% if category.posts %}
    This category has posts
{% endif %}