Translation:Their grandmother is on the load of the car.
This sentence really pushes it in terms of logical and grammatical sense.
The author of this sentence probably intended to express that the grandmother's body weight counts to the car's maximum weight capacity (in the context of a carload of several adults). It's not an adequately structured sentence, neither in Danish nor in English, and should not be on Duolingo. The arrangement of words and phrases to create well-formed sentences in a language, i.e. the syntax, is just as important in language acquisition as the single words and phrases themselves and needs to be given higher priority here in Duolingo.
This sentence reminds me of the old TV show: The Beverly Hillbillies. During the show's theme song, Granny sits in her rocking chair on top of their loaded car, as they move their belongings to Beverly Hills, California.