An aggregate measurement is a model that contains a collection of measures together with their corresponding dimensions. Measures are aggregate numbers, such as Total Sales or Number of Orders. Dimensions are slicers, such as Product, Vendor, or Customer, that help you analyze the measure. For example, the measure of Total Sales isn't useful unless it can be sliced by Product, Region, and Customer.
Reference:
https://docs.microsoft.com/en-us/dynamics365/fin-ops-core/dev-itpro/analytics/analytics