Really simple, your sql has a sub query, something like this
Select * from orders where customer_id =
(select customer_id from customer where name ='bob')
Now the query above will only run providing the sub query only returns one record, eg. there is only one Bob.
If there is more then one Bob there will be more then on customer_id return so the ='s operator won't work (you would have to use in or something).
Paste your query so that we can tell you in which other ways you can write it.