Translation:A vegetarian does not eat meat and fish.
It comes from the Bible, which classifies poultry and fish separately from meat, because it considers that animals have four legs, therefore birds and fish are not animals.
Of course, now we recognise that that they all belong to the animal kingdom, so really they are all animals.
Note that the language is very careful to divide animals from living creatures in the water (fish and other aquatic life such as lobsters and oysters), from birds, from insects, and from other "creeping" creatures (moving along the ground) which would include worms and snails.
The "r" can denote the plural indefinite (menneske (person) - mennesker (persons)), the agent of the verb (fortelle (to tell) - forteller (teller)) or the present tense (fortelle (to tell) - han forteller (he tells)).
In this case, "vegetarianer" is the base form. The plural indefinite would be "vegetarianere".