
Explanation

Box 1: TOPN
TOPN returns the top N rows of the specified table.
Box 2: SUMMARIZE
SUMMARIZE returns a summary table for the requested totals over a set of groups.
Box 3: DESC
Sort in descending order.
It is last in the TOPN command.
TOPN syntax:
TOPN(<n_value>, <table>, <orderBy_expression>, [<order>[, <orderBy_expression>, [<order>]]...]) Reference:
https://docs.microsoft.com/en-us/dax/topn-function-dax
https://docs.microsoft.com/en-us/dax/summarize-function-dax