The Consumer Price Index (CPI) is a way to measure how much prices have changed over time for a bunch of things that we commonly buy. This can help us see if things are getting more expensive or cheaper. CPI is often used to check how much prices are going up (inflation) and how much more we have to pay for the same things we used to buy.

For example, if the CPI is 100 this year and 105 next year, it means that the prices of things we commonly buy have gone up by about 5%. So, if someone bought the same things as they did last year, it would cost us 5% more this year.