Materializing the relation

Hi,

What is the meaning of 'materializing' a relation after sequential scan?



explain select * from tb1, tb2  where tb1.c1 = tb2.c2;

                           QUERY PLAN
----------------------------------------------------------------
 Nested Loop  (cost=1.03..2.34 rows=3 width=24)
   Join Filter: (tb1.c1 = tb2.c1)
   ->  Seq Scan on tb2  (cost=0.00..1.04 rows=4 width=12)
   ->  Materialize  (cost=1.03..1.06 rows=3 width=12)
         ->  Seq Scan on tb1  (cost= 0.00..1.03 rows=3 width=12)


In this above plan, what does 'Material' mean?

Thanks,
Rupesh