This document summarizes key concepts from Lecture 2 of a macroeconomics course. It defines theory as a framework for understanding relationships between cause and effect. Economic models are then introduced as simplified representations of how parts of the economy function, relying on assumptions and economic variables that can take different values. Rational behavior and marginal concepts are explained, showing how rational actors make decisions by weighing marginal costs against marginal benefits. Finally, principles of economics are outlined regarding how individuals make decisions, interact in markets, and how the overall economy works.