An attribute is a special information defined inside an entry of the iCalendar. It contains further information in order to explain better or add more information to the entry...there can be many attributes for one entry! Actually, it's only implemented in the case of a RRULE.
If the iCalendar contains the entry
RRULE:FREQ=WEEKLY;UNTIL=20050314;INTERVAL=1;BYDAY=MO