De jure segregation is applied by law, meaning there's a State actively separating society by race. To give you an example, you can think of how many public bathrooms used to be exclusive for white people by law.
De facto segregation, on the other hand, happens organically or by fact without any external enforcement. To better understand this, you can think of how there's certain neighborhoods that historically have been prominently inhabited by a certain race or another. This prevents other races to settle in those areas as they may feel excluded, prompting a more organic form of segregation that we call "de facto segregation".
Hope this helps!