What does the sli stand for on your car?

It's the indicator of the trim level. Historical examples of these would have been:

G = Grand
D = Diesel
SE= Special Equipment
S = Either Sports, Super or Standard depending on manufacturer
L = lux
i = fuel injection

Example: Ford Sierra 2.0i GLS was a 2 litre, fuel injected Grand Luxury interior with 'sports' trim such as spot lights and front fog lamps when compared with the standard car.

However, as the trend for more letters and numbers on the back of cars grew many of the older naming conventions became obsolete.

SLi on a Rover 400 means two trim levels above the base model - all Rover 400's were fuel injected and the base model was just badged i. Then Si, SLi, GSi