Have a 2003 Audi a6 2.7t quattro, from time to time my cars ABS and ESP light comes on.
If I turn my car off and back on, at a stop light the light will turn off but sometime turn back on.

Please help me out I don't know what it is.

 

If you do a search in the a6 forum part you will find a bunch of threads with the same problem description. Even I have those lights coming on especially in the winter time - and same as you if I shut off the car and turn it back on they are gone (for a while). As far as I know this is most likely by a bad ABS module and you will have to get it rebuild (or buy a new one). There is a "known" group that does that fairly cheap (compared to buying it new) and they even offer more warranty than a new one would give you.
Try them out: ModuleMaster (All ModuleMaster rebuilds come with a 5-year guarantee against module related failures.)

 

I had that light come on last year, turned out to be the brake light switch. They're like 20 bucks and easy to change yourself.