Mild hybrids, broadly speaking, are cars that have both a traditional internal combustion engine alongside an electric machine/generator. This means that the petrol/diesel part of the engine can be turned off at various points during a journey, such as when a car is stationary, but it can then restart really quickly.
A mild hybrid takes the principles of a hybrid, but utilises them in the mildest of ways. They typically have a small battery and an electric motor that can take over running the car’s ancillaries, such as the headlights and air conditioning.
This is particularly useful, because it means the engine can be turned off to save fuel at times when it is not needed, such as when decelerating or when stopped. The important distinction here, too, is that the wheels cannot be powered by the electric motor.
While it tends to mean the engine isn’t being used at times when it’s not being particularly inefficient anyway, the economy gains are fairly small. However, on the plus side, it’s the batteries that tend to add cost to hybrids, meaning mild hybrids tend to be less expensive to buy than other models.