Measures in Power BI document are DAX based aggregations for slicing and dicing the tabular datasets using evaluate and filter context and and data relationships. Short hand, measures are DAX formulas stored in Power BI as part the dataset.
For showing how easy (once you know) is to copy measures between different dataset, I have created a sample Power BI with diamonds dataset (also available on Github).
I’d call the process convoluted. It does work, but it seems like Power BI should have an easier method.