Air resistance is very important consideration for solar cars, or any vehicle that travels at high speeds, since the power to overcome air drag increases cubically with speed. In other words, if you double your speed, the power required to overcome air resistance increases by a factor of eight.
Air resistance is (more or less) the friction which we get from moving through the air. If a car is going faster, then it will hit the atoms and molecules of the air harder. This means that the tiny forces will be bigger. This in turn means that the air resistance force gets bigger as something moves faster.
The air resistance of a vehicle depends on three factors. These are shape of the vehicle, surface finish and speed o the vehicle with respect to the air. Out of these three factors the most important factors is the shape of the vehicle. This is almost entirely determined by the vehicle manufacturer. The user of the vehicle has limited control over this in form of external accessories fitted on the vehicle. Any luggage carrier fitted on top of a car or other vehicle can increase air resistance significantly, particularly if luggage has been loaded. Keeping the windows of the vehicle open amounts to changing the shape of the vehicle, and tends to increase air resistance.
The relative speed of vehicle, particularly at high speed also has significant effect on air resistance. However it is not a practical solution to keep the speed of vehicle low just to reduce air resistance.
A smoother surface offers less resistance to air. Therefor, one can keep the vehicle well painted and polished to keep the air resistance low.