There are two distinct ways of commenting Haml markup: HTML and Ruby.
This will create an HTML comment that will be sent to the client (aka browser):
/= link_to 'Example', 'www.example.com' # => <!-- = link_to 'Example', 'www.example.com' -->
Only use this variant if you need the comment to appear in the HTML.
This will comment code so it will not be sent to the client:
-# = link_to 'foo'
99% of the time you'll be adding notes for other developers, or disabling code sections that should not be sent to the client.
-# (which is actually:
- "start a Ruby block" +
# "Ruby comment").
This type of comments needs to be indented properly. If you get a syntax error (sometimes a very unhelpful one, echoing the whole file without giving a specific line number), don't forget to check the comments.