This is the 9547th most frequent Russian word.
фитнес
Fitness.
In this sentence, 'фитнесом' is used as a noun in instrumental case to describe the activity that someone engages in.
Я занимаюсь фитнесом три раза в неделю.
I do fitness three times a week.
Here, 'фитнес-клуб' combines the noun 'фитнес' with 'клуб', forming a compound noun that refers to a specialized venue for physical exercises.
Мой сосед открыл новый фитнес-клуб рядом с домом.
My neighbor opened a new fitness club near the house.
In this instance, 'фитнес' is a noun in its base form used as the subject of the sentence, signifying a general concept or practice.