Probably "Sport" mode? I don't know Jags, so I'm guessing a bit here, but Sport mode is common in these nice cars--it changes the engine mapping to be more aggressive and performance-oriented. If you have the owner's manual, it should tell you.
it's for sport mode. The transmission will shift a sooner and sportier with the s lite on. Your mileage will be a bit less, but the car is funner to drive in the sport mode. I leave mine in sport all the time.