People (especially men) sometimes say "Hey" before they respond to what someone has said. For example:

A: You're one of the smartest people I know.

B: Hey, I appreciate that. I don't know if it's necessarily true, but it's nice of you to say.

A: Well listen, I have to get going. I've got a meeting in a few minutes.

B: Hey, I understand.

There are a couple of reasons for it:

• It fills a little bit of time and gives the speaker longer to figure out what to say.
• It makes the speaker sound friendly and easy-to-talk-to.