a scantool is your tool to look into the cars computer's brain, the ecm. Usually you need a laptop and a program such as turbolink or several others offered to do such a thing. You just plug into your diagnostic terminal near your ash tray and read the information the program gives you and you can adjust and tune your car from that information.
It really helps to see where your having problems, such as running rich, vacuum leaks, transmision shift points, etc........
but first you gotta understand what all that information is. at first glance it looks really greek. Read around here and under the scantool topic to find out more.
