In automotive use, g is most commonly used as a measure of lateral acceleration, ie cornering. It can also be used as a measure of acceleration in any other direction.
An acceleration of 1g is equivalent to the acceleration due to gravity, which is 9.81m/s2 (32ft/s2). If you drop an object, after one second it will be travelling at 9.81m/s, after two seconds twice that and so on (ignoring air resistance). That is an acceleration of 1g. An acceleration of 2g is twice as much.
The best road cars can generate 1g or more in cornering, braking and forward acceleration. F1 cars corner and brake at up to 6g, while an aerobatic or fighter pilot could experience up to 12g in tight turns.
By contrast, the average road driver rarely exceeds 0.2g in any direction.
