
Explanation

Box 1: CALCULATE
Start with CALCULATE and use a SUMX.
CALCULATE evaluates an expression in a modified filter context.
Box 2: SUMX
SUMX returns the sum of an expression evaluated for each row in a table.
The following sample creates a measure with the sales of the top 10 sold products.
= SUMX(TOPN(10, SUMMARIZE(Product, [ProductKey], "TotalSales",
SUMX(RELATED(InternetSales_USD[SalesAmount_USD]), InternetSales_USD[SalesAmount_USD]) + SUMX(RELATED(ResellerSales_USD[SalesAmount_USD]), ResellerSales_USD[SalesAmount_USD])) Box 3: TOPN TOPN returns the top N rows of the specified table.
Box 4: [TransactionDate]
TOPN Syntax: TOPN(<n_value>, <table>, <orderBy_expression>, [<order>[, <orderBy_expression>,
[<order>]]...])
The orderBy_expression: Any DAX expression where the result value is used to sort the table and it is evaluated for each row of table.
Reference:
https://docs.microsoft.com/en-us/dax/topn-function-dax