A census metropolitan area (CMA) is one or more adjacent municipalities centred on a core population centre. Generally, if you live in, or close to a major city, you are in a CMA.
If you live in a province that offers the climate action incentive, TurboTax will give you examples of what is in the CMA(s) for your province. If you are not sure if you are in/outside of one, this CRA page has maps of those regions: https://www.canada.ca/en/revenue-agency/services/child-family-benefits/cai-payment/qualify-for-the-supplement.html