This is the 9562nd most frequent Russian word.
фундук
Hazelnut.
Here, 'фундук' is used as a noun to refer to the nut that is consumed as food.
Я очень люблю фундук с медом.
I really love hazelnuts with honey.
In this sentence, 'фундук' is again used as a noun to emphasize its nutritional value.
Фундук — это полезный орех богатый витаминами.
Hazelnuts are a healthy nut rich in vitamins.
Here, 'фундук' is referred to as a plant/tree that yields the hazelnuts.