What's it: NOTICE: Adding missing FROM-clause entry

Hi All! :)
There are two tables:
test(a int4,b varchar(20));
test2(c int4,d varchar(20));
I do the following:
select a,b,test2.d from test where a=test2.c;

and i receive:
****
NOTICE: Adding missing FROM-clause entry for table "test2"
a | b | d
---+---+---
*result*
****
Question: what's this- NOTICE: Adding missing FROM-clause entry for table
"test2"

version: PostgreSQL 7.1

Thanks..

Sorry for my english...:)