Or similar - What does it mean?

The term "Or similar" indicates that the vehicle you rent may not be the exact make and model as the vehicle displayed, although it will be from the same group, meaning, that it will comparable,  in size and performance to that vehicle. 

The actual vehicle you rent will depend on the makes and models available at the time the car group you request. 

If we are unable to provide a vehicle in your requested car group, we will upgrade your rental car from the next available car group at no extra charge.