The answer depends on what you need to do and what range of Fabrik's functionality you need to use:
For the basic use of Fabrik no coding is necessary, so for example you can create a form which records data into a list, and then create filters for that list to search for particular records without any programming (PHP, HTML, CSS) knowledge at all.
The next step up would be wanting to style the look of your lists or forms, in a different format to the default templates that come with fabirk. This requires some html and/or css knowledge.
Within the list functionality there are some advanced concepts (that you might not ever need to use) that require an understanding of how sql works. An example of this would be joining your list's data to your site's Joomla users table. Whilst this doesn't require any coding knowledge an understanding of how this works in SQL would be helpful.
Then there are validations (checks on the form data submitted), out of the box you can choose to check if a field is empty, or if it is an email address. If you were looking for something more specific then you would have to write a regular expression for that yourself (or ask on the forum).