The answer to this is "charge more". You should be pulling in _at least_ 2 hours worth of payroll costs (note: not 2 hours of pay) for each worked hour. This give you a buffer for lulls.
They mean the costs a company would bear for you if you were a full-time employee. This includes your theoretical employment salary plus extra costs an employer pays for having that employee on their books, for example in the UK there is employers national insurance (tax), as well as other overheads like a fractional contribution to office space and software licenses.