The words मतलब and अर्थ mean… well… “meaning”.
It is very common in Hindi to simply say “मतलब” to mean “what do you mean?”
मतलब? – “what do you mean?” / “meaning…?”
If you want to say “what do you mean by X” or “what does X mean”, you can simply say “X मतलब?”.
X मतलब? – “what does X mean?” / “what do you mean by X?”
To ask “what does X mean” explicitly, say “X का मतलब क्या है”.
X का मतलब क्या है – “What does X mean?”
To say that something “doesn’t make sense”, either मतलब or अर्थ can be used:
असम्भव प्रयास करने का कोई मतलब नहीं है – “I makes no sense to attempt the impossible!”
These words can be used to say “it doesn’t mean that…”:
सिर्फ इसीलिए कि मैं आपके जितना पढ़ा नहीं हूं, इसका मतलब यह नहीं है कि मेरी बात गलत है – “Just because I’m not as educated as you doesn’t mean I’m wrong!”