When へ is used as a response, to something someone has said, what does it mean?

Question

I have heard the expression used, but can't find its correct spelling or usage.

Answer 1

へえ (also spelled へぇ, へー, へぇ～, ...) is even listed in a dictionary (from 大辞林):

へえ [1]
（感）

1. 驚いたり、感心したり、疑ったりした時にいう言葉。「—、彼が結婚したとはねえ」「—、本当かね」

2. （主に関西地方で女性が）応答・承諾などに用いる語。「—、おおきに」

roughly

へえ
interjection

1. An utterance expressing surprise, admiration or doubt. »—, so he got married, eh?« »—, really?«

2. A word used (especially by women in the Kansai area) to reply or agree. »—, thanks a lot«

Depending on the context, English equivalents could be "wow", "oh", "no way", "huh", etc.

Answer 2

If you mean a more or less prolonged へー , often with a falling-then-rising intonation, it indicates that the person using it finds what has just been said surprising, remarkable or impressive in some way. Something like "Good Heavens", "You don't say", "That's amazing", "Wow". Used more by female than by male speakers.