I got a SES light, which comes up as code p4001 i think. Basically it said "secondary air injection system failure"

**I did notice my car constantly choking when i drive real slow, like in traffic for instance, and when I back up is stalls when i give it gas and drop the pedal to brake.... same when going straight is stalls too.

1. What is "secondary air injection system"?

2. How do I repair this, what do I repair?

Please help ASAP!