You could evalute this and see this in Excel, there is an button shows "analysis in Excel"
For your expression, you could see similar scene at this article which will tell you clearly how to use the expression : DAX COUNT and COUNTX – Impact of Using Measures V’s Calculated Columns
As DAX documentation, normally you could find examples and guides at microsoft's documentation. https://learn.microsoft.com/en-us/dax/
If you want to learn DAX systematically, I would recommend the dax bible : The Definitive Guide to DAX by the very dax creator. You could easily google this book.
regards,
Lukas