Hi Kevin and welcome to the QlikView world,
The whole expression uses what is known as "set analysis" all but the Sum() and the Salary, and reads as follows: Given the current state of selections of the document, sum all values in field Salary for those records that do not have in the whole data set the field IsActive equal to 0.
Not knowing the example, it sounds like if he is summing up all the amounts for those employees that are active (IsActive = 1 or anything but 0), but not those that might be active in the past, but they are no longer active (IsActive = 0).
The element function E() returns a record set with all values excluded by the given modifier (in this case, IsActive = 0).
Hope that helps.