Pardon?
What it really means
The polite way to say "I did not hear you, please repeat". Softer than "What?", which can sound rude in Britain. "Sorry?" works the same way.
Context, tips & common mistakes
For 'I didn't hear you', the frictionless choice is "sorry?" with rising intonation — universal, classless, always right. "Pardon?" does the same job and is entirely polite in most company, though etiquette trivia fans should know it sits oddly in the British class labyrinth: some circles consider it genteel, others (counterintuitively, often the poshest) prefer a blunt "what?" — which from a learner can sound rude. Stick with "sorry?" and you never have to think about it again.
The stressed, theatrical version — "I BEG your pardon?" — is not a hearing problem but offence taken, demanding you retract or explain what you just said. And when you mishear repeatedly, upgrading to "sorry, I didn't catch that — could you say it a bit slower?" is welcomed; pretending to have understood, then guessing, is the option that actually annoys people.
Example conversation
What you could say back
No spoken reply needed for this one.