An anti-lag system is an automobile engine/exhaust modification designed to combat turbo lag.
Turbo lag is caused because the turbines in the turbo system take time to spin up to speed. This lag is magnified in racing engines that have large turbos.
The older anti-lag system worked by injecting fuel into the exhaust system, upstream of the turbo, when the throttle was released. This injection of fuel into hot exhaust results in ignition of the fuel in front of the turbo. This causes the turbines to keep spinning at high speeds even when the engine wasn't.
Later systems using more advanced computers to control the engine simply altered the timing of the engine so that fuel ignition occurs when the exhaust valve is open, with the same results.
The drawback to this system is that the exhaust temperatures are much, much higher than normal (~1100 degrees C vs. ~800 degrees C). This results in greatly reduced lifetimes for the turbo and exhaust components, which is fine for rally cars in races, but unacceptable for street vehicles.
Another potential problem is the huge amount of pops and bangs and gouts of flame shooting out the tailpipe. Cool, if you are into that sort of thing.