To make that a litle clearer - Maybe you want to study a certain subject. There is only one place that offers this subject, but there is a rule which says that to study on this course you first have to take and pass an examination in the subject. The rules for eligibility to enter for the examination say you must have first completed a recognised course. This is a catch 22 - you can't do the course without passing the examination but you can't take the examination without first doing the course. It's a paradoxical situation.
Hope that helps