Someone is a 'hack' when they produce low-quality, often sensationalized work that doesn't have much depth or meaning. It was originally a term to describe writers who were willing to write literally anything for money. For example, many people have called people who write for buzzfeed 'hack writers.' Someone might call someone who works for a tabloid a 'hack journalist.' We see someone as a hack when they're in a profession we think should be handled with care, but their goal isn't to do a good job; it's to make money or stir up drama or attention.