What is the 2% rule in real estate?

Excuse me, as a professional in the field of finance, I've often heard of the "2% rule" being referenced in real estate investments. Could you elaborate on what this rule actually entails? I'm curious to know if it refers to a certain percentage of a property's value that should be used as a guideline for rental income, or perhaps a threshold for the amount of a mortgage payment that should not exceed a certain percentage of one's income. Your expertise in this matter would be greatly appreciated in clarifying this seemingly popular but sometimes ambiguous real estate guideline.