"Euro Plus Garantie" is the same as you will get from bmw bying a used car.
If we talk about a one year old car - a new car is sold with two years of warrantee from bmw so there is still one year left. Maybe in the add this is meant by garantee - Maybe the car owner has bought another (third) year of guarantee. I am not quite sure, you should ask the owner of the car what it is about.