@EmmaSmoke stole this definition from a friend
"Basically, an Apply-Fic is short for Apply-Fiction. Meaning, its plot is based on a fictional story or its plot idea, despite being ORIGINAL, is still a fictional story. TL;DR: part of the fiction genre.
Given the name APPLY, the creator makes the plot and a certain amount of cast members/characters. Then, the creator makes a form. This form typically contains bio information, such as age, gender, face claim for the character, etc.
This is so that people can make their own characters and 'apply' in order to have a chance to have them included into the story."