Ultimately, safe is what we are all craving, and what we all strive to find. In the classroom, if a teacher is safe, it means she's consistent, and that she is predictable. If a parent is safe, it means they are the same.
It does NOT mean that the adult is indulgent. Indulgent does not make the child feel safe. In fact, it often feels the opposite. Sometimes indulgence codes in as indifference.
What makes us feels unsafe is changing rules in mid-stream, or inconsistent rules. Something that is a rule one day and not the next for instance, only makes us feel unsure, and unsure means unsafe.
One of my son's teachers would yell at the children if anyone dared to speak during prayers. She would be furious and would rail about being disrespectful. (Catholic school) Then later in the same week, she laughed because she forgot the words.
Consistency means the adults operate out of the same rule book. If you don't want the bedroom door locked when they are teens, then don't lock them out when they are little. They will model what you do and not what you say.