What is Oracle's Window Buffer Operator?

Question

Oracle database's explain plan sometimes includes Window (Buffer). What does this operator do?

Here is the SQL. The primary keys are RECORD_DATE, TYPE and KEY.

select
   source.RECORD_DATE,
   source.TYPE,
   source.KEY,
   source.AD,
   source.VALUE,
   source.STATE,
   source.SUB_STATE_KEY,
   source.AMOUNT
from
   COMPUTE_DETAIL_FACT source
where
   source.RECORD_DATE >= date '2018-12-09' and
   source.RECORD_DATE <  date '2019-02-25' and
   source.TYPE        =  'banana' and
   source.KEY         =  316598 and
   not exists
   (
      select
         0
      from
      (
         select
            lead(test.RECORD_DATE) over (order by test.RECORD_DATE) RECORD_DATE
         from
            COMPUTE_DETAIL_FACT test
         where
            test.RECORD_DATE    >= date '2018-12-09' and
            test.RECORD_DATE    <  date '2019-02-25' and
            test.TYPE           =  source.TYPE and
            test.KEY            =  source.KEY and
            (test.AD            =  source.AD            or (test.AD            is null and source.AD            is null)) and
            (test.VALUE         =  source.VALUE         or (test.VALUE         is null and source.VALUE         is null)) and
            (test.STATE         =  source.STATE         or (test.STATE         is null and source.STATE         is null)) and
            (test.SUB_STATE_KEY =  source.SUB_STATE_KEY or (test.SUB_STATE_KEY is null and source.SUB_STATE_KEY is null)) and
            (test.AMOUNT        =  source.AMOUNT        or (test.AMOUNT        is null and source.AMOUNT        is null))
      ) prev
      where
         source.RECORD_DATE = prev.RECORD_DATE
   )