"Sum" is the function name. the "(" and ")" mark the start and end of the passed parameters to the function. the "n1 [,n2...]" indicate that there is one required parameter and then any number of optional parameters. The description in the 'Scripting Reference" under LiveCycle Designer's "Help" is:
"Returns the sum of the non-null elements of a given set of numbers."
So one could enter numbers, the values form field names, or an array of a field name as parameters and get the sum of the values.
If I have a row of fields called "DailyHours" and have 7 of these fields I could code for the "Calculation" script for the row total using FormCalc: