Why can't I add a script to an object?

I made this script called, Skid Audio, and try to drag it onto a car object, but it says this,

Script Skid Audio has not finished compilation yet. Please wait until compilation of the script has finished and try again.

What does this mean and how can I get the script to work?

It means that when you save a script you can see in the right bottom corner a little spinning wheel, as long as the wheel spins, mono develop (or else) and unity are linking the data. So you need to wait.Once the wheel is gone, no porblem.