Haha! I love that I get to answer this question.
A basic bitch is someone with basic interests. She likes boring common things, has no ideas in her head, and has no need for adventure. She just follows trends and celebrities and says what everyone else says like its the smartest thing in the world. If you don't want to use the word bitch you can just say that someone is basic!
Don't be basic! Keep studying English.
Blake