a reference to seq_name.NEXTVAL is used to get the next value in a sequence. A single statement can only generate a single sequence value. If there are multiple references to NEXTVAL in a statement, they use will use the same generated number.
NEXTVAL can be used for INSERTS
INSERT INTO Orders (Order_UID, Customer) VALUES (orders_seq.NEXTVAL, 1032);
It can be used for UPDATES
UPDATE Orders SET Order_UID = orders_seq.NEXTVAL WHERE Customer = 581;
It can also be used for SELECTS
SELECT Order_seq.NEXTVAL FROM dual;