An IAC stand for idle air controller. What it does is when you let off the gas (the TPS reads no throttle) the computer takes over the airflow through the throttle body to bring the engine back down to idle, and then idle the motor.

 

ok. so then, if i start my truck, and it wont stay running unless i keep my foot on the gas, this IAC could be my problem?

ALso, how would i now if its the transmission giving me these problems. I tried to get the truck moving by keeping on the gas and shifting into gear, but it just dies again.