What is a primary (PRE) fuel filter?

A Pre-filter should be there to do exactly what its name says. Filter the fuel ‘pre’ the main filter. Why? If the fuel is in a poor state (contaminated with gunk/algae and other matter) it will have some of that removed so that the MAIN filter is left to worry about filtering the remaining stuff. The main filter will block quickly with a bad batch of fuel and leave you with a car stranded in limp mode. The pre filter is there to try to stop this problem protecting the OE main filter from the worst of a very bad fuel fill