Think of it like this. If you need to look up Pfaffman in a phone book, you go to the P, then try to find entries between Pe and Pg, then between Pfa and Pfb etcetera. You use the fact that the entries are sorted alphabetically to quickly look up the entry. Now imagine - as a mind experiment - that one of the phonebook editors didn’t memorize the alphabet correctly and inserted some Pfaffmans at the wrong place (a corrupt index). You won’t find them.
Now you need to look up all people with a last name ending with faffman. There is no fast way to do that, you will need to go through the entire phone book. That’s a lot of work! But… you will find the incorrectly inserted Pfaffmans this way!