Black folks often use the N word casually to refer to each other as a form of taking back the word’s meaning. It used to be used exclusively in a racist fashion. The primary difference is that with the African American accent, the ending sound -ER is changed to more of an -UH sound. Often times, rarely and depending on the context, it is allowable for non-black people to say it with this accented pronunciation. But under no circumstances is it in good taste to use the original -ER ending to refer to a black person as a non-black person, that form is only used as a slur. When people refer to the “Hard R”, this is what they are talking about, the difference between the accented pronunciation as slang vs the original pronunciation intended as a slur.