A rule of thumb is a guideline, idea, or principle that helps you make decisions. "Arrive early" is a good rule of thumb for most appointments.
This term originally referred to builders who used their thumb to estimate measurements. The meaning broadened to mean any inexact but helpful rule. "Be specific" is a rule of thumb for most types of writing. "Look both ways before crossing the street" is a rule of thumb for pedestrians. If something is an official or written rule or law, it isn't a rule of thumb. Rules of thumb are unwritten but helpful guidelines. Everything, from parenting to architecture, has its rules of thumb.