Wahey, the Vauxhall ECU error light! Was my best friend for a while when I had my 1995 Cavalier.
It means something has gone wrong somewhere, and the car is in "limp home mode". You'll probably find that it is giving really bad fuel economy with it on, not accelerating very well, and if you try and rev the engine higher than 4000rpm, it won't let you - will feel like hitting a brick wall as the engine stops revving.
It is usually a sensor fault that causes the ECU light to come on. It was the crank sensor first on my cav, then the cam sensor a few weeks later. You can take your car to a garage, who will charge you £50+ to plug the ECU into a machine, or you can find the plug in the engine bay, stick a paperclip across two specific pins and that orange light will flash a certain number of times, giving you an error code. You can then read the error code up on a chart and see what sensor is giving you an error message.