A sequence is a auxiliary relational data structure that maintains a sequence.
Relational sequences are generally used to create a generated primary key via an incremental number.
They may also be used transparently when a auto-increment instruction in SQL is given on a column
For example with SQlite:
CREATE TABLE foo (
id INT autoincrement,
description TEXT
);
Database may give you the possibility to query the actual or next value of a sequence.
Example with an Oracle sequence.
select my_sequence.nextval from dual;