The full phrase is "(up) in my grill" = "(up) in my face". These are both slang phrases that means that someone is being very annoying, hostile and/or bothersome to you as well as getting too close physically. "He started yelling at me for being late and got all up in my grill about it."

You mean the Missy Elliott song where she sings, "Why you all in my grill?" Basically, it means "why are you here, annoying me so much?" "In my grill" = in my face (the frontmost part of a car is the grill). If you're "in someone's face", you're deliberately annoying that person. In your quote, "all" means "completely".

Ha ha. Do you just want to understand Missy Elliot's song or are you studying to be a rapper? I would say "Why are you confronting me?" or "Why are you being confrontational?" is a good translation. In the chorus of the song, I think Missy Elliot is saying, "Why are you telling me what to do? You don't pay my rent. But can you? Will you? You should be paying." Although I think some songs are excellent for helping an English learner with pronunciation and grammar, other songs are abysmal, especially rap, or in this case, a rap with a little melody. From an English teacher's viewpoint, the language in this song is a mess!