What does it mean by "PROC SQL makes two passes through the data" ?

[shihabur]

I am reading a PROC SQL tutorial and got stuck up in these following lines - 

" When processing a set operation that displays only unique rows (a set operation that contains the set operator EXCEPT, INTERSECT, or UNION), PROC SQL makes two passes through the data, by default. For set operations that display both unique and duplicate rows, only one pass through the data is required "

So what does it mean by the term "Pass through the data" ? Any relevant link or explanation or even brief clue will be highly appreciated. 

[Kurt_Bremser · Accepted solution]

It means that the dataset will be read twice, causing more I/O and costing more time.