[oracle] When or Why to use a "SET DEFINE OFF" in Oracle Database

I'm watching a Script in Oracle and I see something I don't recognize

REM INSERTING into database1."Users"
 SET DEFINE OFF;
Insert into database1."Users" ("id","right") values ('1','R');

I'm looking for documentation about "set define off" and it's literally writing "disable the parsing of commands to replace substitution variable with their values"

I don't really understand what they want to say.

Can anyone help me?

Here is the example:

SQL> set define off;
SQL> select * from dual where dummy='&var';

no rows selected

SQL> set define on
SQL> /
Enter value for var: X
old   1: select * from dual where dummy='&var'
new   1: select * from dual where dummy='X'

D
-
X

With set define off, it took a row with &var value, prompted a user to enter a value for it and replaced &var with the entered value (in this case, X).