It's a sort of dummy table with a single record used for selecting when you're not actually interested in the data, but instead want the results of some system function in a select statement:
select sysdate from dual;
The DUAL table is a special one-row table present by default in all Oracle database installations. It is suitable for use in selecting a pseudocolumn such as SYSDATE or USER
The table has a single VARCHAR2(1) column called DUMMY that has a value of "X"
You can read all about it in http://en.wikipedia.org/wiki/DUAL_table