Logic is, essentially, deductive reasoning. Given certain information, certain conclusions must be true or untrue.
It is logical, for instance, to assume that Harry Potter and Draco Malfoy are schoolyard enemies, because they are frequently in competition and have mutually abrasive temperaments, because they were raised with different sets of values, and because they tend to antagonise each other simply for the sake of it.
It is not logical to assume that Harry Potter and Draco Malfoy are clandestine lovers because they are both 'hott'.
Logic is something Suethors and badficcers lack. When setting up a plot, it helps for it to involve logic somewhere. If your plot is sans logic, however, do not be surprised if the PPC come knocking on your door.