DDL With the WAIT Option

The DDL_LOCK_TIMEOUT parameter indicates the number of seconds a DDL command should wait for the locks to become available before throwing the resource busy error message

For Example :

CREATE TABLE lock_tab (id  NUMBER);
INSERT INTO lock_tab VALUES (1);
ALTER SESSION SET ddl_lock_timeout=30;
ALTER TABLE lock_tab ADD (description  VARCHAR2(50)); 

ALTER TABLE lock_tab ADD (
ERROR at line 1:
ORA-00054: resource busy and acquire with NOWAIT specified or timeout expired

 Ref :

Thank you
Osama mustafa