Why is my Audi A3 showing an engine error code P0171?

I recently noticed a check engine light on my Audi A3 and my mechanic found the error code P0171. What does this code mean and how can I fix it?

The error code P0171 indicates a lean air/fuel mixture in the engine. This can be caused by a dirty or faulty mass airflow sensor, a vacuum leak, or a clogged fuel injector. I recommend taking your car to a certified Audi technician for a diagnostic test to determine the exact cause of the issue.