Mine specifically? "I helped two million kids learn to read last year" or "Occasionally, companies fly me out to them so that I can work for a week or three and make them several million dollars. The other 40 weeks of the year I keep myself amused."

Back when I was doing software development every day, the answer was something closer to "You know X university? I make sure their entrance exams run smoothly."

For a generic software developer, "I make my employer absolute piles of money by being clever in the general vicinity of a computer."