1990 Chevy Corsica Idle above 1100 RPM

I just bought this car and an emissions test is required to transfer the title over. I was unable to do the test however because the car's idle is too high...whatever that means. I was told to take it in somewhere to get the "idle set below 1100". What does this mean exactly? Isn't the onboard computer supposed to regulate this? I have limited funds at the moment and want to get as much information as possible before I take it in to a mechanic so that I won't be taken advantage of.

Thank you!