Sumx is the x version of the SUM function, this function takes two parameters.
The first parameter is a table name or any function which returns a table. And the second parameter is an expression to evaluate while this can be a column that need to be aggregated.
This function creates a row context in the given table and iterates over each row and evaluates the expression in the table.
SUMX run thru all the rows of the table and evaluates the expression, finally it aggregates all the values in the expression.
Eg: =SUMX(Sales, Unit Price * SalesQty)
Please watch the video below to understand more on this..