What is an unfit parent?

Basically someone who neglects or abuses their kid(s)

Someone who doesn't care and doesn't pay attention to their kids

"A parent may be deemed unfit if they have been abusive, neglected, or failed to provide proper care for the child. A parent with a mental disturbance or addiction to drugs or alcohol may also be found to be an unfit parent. Failure to visit, provide support, or incarceration are other examples of grounds for being found unfit."

The legal definition varies state to state, but the above is a good summary.