CFM just means cubic feet per meter, and it is just a unit that is used to measure the volume of air that is exchanged in a single minute over a certain surface of the area. This determines the power of suction in your exhaust fan as the higher the CFM is, the more powerful the suction power of the exhaust fan would be and the larger of an area the exhaust fan would cover.    So if instance your bathroom’s floor has an area of 60 square meters, you would not need an exhaust fan that has CFM higher than 60, as to install an exhaust fan, the CFM is calculated comparing it with the area of the floor, because for 1 square meter, 1 CFM of an exhaust fan is required.