This idiom is always presented in a negative way, and most often as a threat. If a person is physically violent, he or she may hit, slap or punch others, using his entire hand. What this idiom is saying to such a person is that using even one finger in an aggressive manner is unacceptable. To "lay a finger on" simply means to touch. So, this idiom is a warning that any physical action whatsoever—even a gentle touch —will not be tolerated. For example, we can imagine two people arguing, and one of them becoming so frustrated that he wants to hit or break something. That is the kind of situation where the second person might speak this idiom to the frustrated one, warning that he'd better limit his aggression to words only, and never touch.