does that mean the temprature at alt 3800feet is 9 degrees, or is 9 degrees higher than the ground temprature, or 9 degrees higher than the temprature should be at 3800feet?
OK465 already gave you the correct answer to your question. It is a relative rise to the OAT - usually given in realtion to the temperature on GND, since only few pilots monitor the OAT during climbout.
To give you an example, let's say the OAT on GND is 15°C, so with the information given on the ATIS you would expect it to be at 24° at 3.800' instead of the ~8° it would be in an ISA Standard Atmosphere.
But don't overdo this whole theory thing. As
OK465 also correctly said, the information about temperature inversions is usually coming from PIREPS, not from the MET office and is given as a warning for departing pilots (a temperature inversion is much more interesting for the departing pilot than it is for the one on descend for approach since the latter will experience an increase in engine performance, while the one on departure will have a decrease in engine performance).
So the only thing I do when this information is given on the ATIS is to keep in mind that I have to expect a decrease in climb performance due to the higher temperature. And if you are already flying on AP (I am usually still flying manually at that alt.), you should be prepared to take over and decrease the pitch slightly, since some AC APs are a bit slow to react to a sudden temperature inversion during climbout.
Cheers,
DBate