Wow, OK, where to start... Truly, songs by Rihanna are not the best way to be learning English. These songs are full of urban slang, regional dialect and incorrect grammar and will get you precisely nowhere in your attempts to learn English well. Yes, 'y'all' means 'you all' but its usage is basically restricted to the south of the United States. You should not attempt to use it. 'Frontin'' is slang. Again, avoid it. I'm not even 100% sure what it means. I think it means 'showing off' but I could be wrong.

Question correction: Why does Rihanna sing "who y'all think y'all frontin on" in this song? Please pay attention to question forms - this is elementary English.