The function QTR returns the calendar quarter from a SAS date value. Qtr 1 is Jan, Feb, Mar, Qtr 2 is Apr, May, June; Qtr3 is Jul, Aug, Sep and Qtr 4 is Oct, Nov Dec.
Since you only have one record with a date for each month Qtr is ge 3 for Jul through Dec.
The IF without a then is a subsetting if and only records where the if condition is true are kept in the output data set.
Does this answer your question? If not please expand on what is confusing.