SQL & PL/SQL

Hi,
Can anybody point me waht is this plus sign means at the end of WHERE:

 select t1.item_cd, t1.sub_item_cd, decode( t2.sub_item_cd, NULL, 1, 0 )
     from t1, (select distinct sub_item_cd from t2 ) t2
     where t1.sub_item_cd = t2.sub_item_cd(+)

I see the difference, but could not find description how it works.

Tx
T

====
item_cd sub_item_cd

1010 A
1010 B
1010 C
1011 A
1011 D
1012 A

rows of second table

sub_item_cd

A
A
A
B
B
C
B
C