Notice how the article “a” reduces to /ə/, and the /w/ sound you need at the end of “you” because the next word begins with a vowel sound.
It may seem strange to be stressing the preposition here, but this is common. “On” here means “talking”. The phrase means “I don’t understand you”, or “I think you’re talking rubbish”. Be careful, it’s rude, unless the person knows you’re joking.