i don't know how to respond to that
This is when you're either too confused or too lazy to say anything. You just throw this out there like you're trying to make the conversation end faster than a bad movie.
'I don't know how to respond to that', said by your brother when you told him you're moving to another state.
'I don't know how to respond to that', said by your ex when you said you're getting married next week.
'I don't know how to respond to that', said by your best friend when you told them you're quitting your job.