What does "nearside" and "offside" mean?

"Nearside" means the side of the car nearest the kerb when driving, which is the left side in the UK, since we drive on the left. "Offside" means the side nearest the middle of the road or oncoming traffic, which is the right side.

These terms exist because left and right can be ambiguous, depending on which way you're facing when you describe a car. Nearside and offside always mean the same thing regardless of which direction you're looking at the car from, which is why testers and garages use them instead of "left" and "right".