Add Column > Custom Column adds a column whose value is worked out for every row by a formula you write in M, Power Query's language.
Inside the formula, a column of the same row is written in square brackets: [Qty] * [Unit Price]. Double-click a name in the Available columns list to insert it. M functions are grouped by what they work on, such as Text.Upper, Text.Start, Date.MonthName and Number.Round, and their names are case-sensitive: text.upper is an error.
Unlike an Excel formula, a custom column has no cell references and no fill handle: the same formula runs on every row, including rows that arrive with next month's file.