The phrase "مش عارف شو بدك تقول" translates to "I don't know what you're saying" and it is considered more informal or casual. If you want to be more polite and respectful, you can say "معذرة، لستُ متأكدًا مما تقول" (Excuse me, I'm not sure what you're saying) or "ممكن توضح أكثر؟" (Could you please clarify more?). These phrases convey your confusion in a polite and respectful manner, showing your willingness to understand the person's message better.