A sequence generates a series of numbers.
A sequence contains two pseudo columns named currval and nextval.
Before retrieving the current value you must initialize a sequence by retrieving the next value.
When you select test_seq.nextval the sequence is initialized to 1.
SQL> CREATE SEQUENCE test_seq;
Sequence created.
SQL>
SQL> SELECT test_seq.nextval FROM dual;
NEXTVAL
----------
1
SQL>
SQL> drop sequence test_seq
2 /
Sequence dropped.