CPI, or Consumer Price Index, is a way of measuring how much things cost for people to buy. It looks at the prices of a group of things that people usually buy, like food, housing, and transportation, and how those prices change over time. When prices go up, we call it inflation, and when prices go down, we call it deflation. CPI is important because it helps us understand how much things are costing people, and how that affects the economy.