What is the meaning of 'He who neglects to assist his relatives may become a slave to an outsider'?

It's about karma; what goes around comes around. If your parent(s) get(s) to the point at which they can no longer take care of themselves, and you fail to step up and make sure they receive the best care you can afford, don't be surprised when you, too, receive inadequate treatment from strangers when you can no longer take care of yourself.