Understanding the difference between a row context and a filter context is the first and most important concept to learn to use DAX correctly. This video introduces the filter context.
Article and download: https://sql.bi/747800?aff=yt
How to learn DAX: https://www.sqlbi.com/guides/dax/?aff=yt
The definitive guide to DAX: https://www.sqlbi.com/books/the-definitive-guide-to-dax-2nd-edition/?aff=yt