The IAC (idle air controller) is what provides all of the engine's air when at idle RPMs & you're not stepping on the gas. It has a solenoid actuated plunger that moves in/out of a small hole in the throttle body to allow more/less air into the engine as controlled by the engine computer.
When it gets dirty, it can hang and cause low or high idle RPM problems. Cleaning it with some aerosol Throttle Body Cleaner and a toothbrush will normally restore it back to good operation. Clean the oriface its plunger fits into in the throttle body with cleaner & a baby bottle nipple brush.
It is only held on by a couple torx screws, you can complete this easy job in 15 minutes.