It changes the subsitution variable indicator to a '~'.

The default is '&'.

So without the command, if you try this :

select 'a & b' from dual;

you get this :

enter value for b :

but after the command, you get this :

'A&B'
-----
a & b

and now, if you issue

select 'a ~ b' from dual;

you get

enter value for b :

hth

-John