Roy's safety-first criterion is a way of thinking that helps people decide if a certain choice or decision is the right one. When using the Roy's safety-first criteria, people should always make sure their decision will keep them safe and out of danger. This means that people should avoid doing anything that could put them in harm's way, like climbing a tall tree or walking through a strange place at night. The idea is that it's better to be safe than sorry, so it's worth taking a few extra steps to make sure you're making the safest decision.