A "fix it fic" is a fanfiction where the problems are solved. Characters who were originally dead in the actual books, shows, or movies, are now alive and well... catastrophes are prevented by the author of the fanfiction and (almost) everyone gets the happy ending they deserved, but never got in the orignal work.
In a Harry Potter "fix it fic" that I read, Cedric Diggory, Sirius Black, Fred Weasley, Remus Lupin, and Nymphadora Tonks all lived to be happy and safe like they should have in the books!