"Who sits on Tono's side?"
Translation:Siapa yang duduk di samping Tono?
The sentence is fine without 'yang' but varies in emphasis. 'Yang' has a range of uses in Indonesian, one being to emphasise the adjective or verb that follows. E.g. Saya memberi mobile baru = I bought a new car (does not always mean the car is new ... just the purchase). Saya memberi mobile yang baru = I bought a car which is new.
So, in their sentence, 'yang' is used to emphasise that the question is about the person 'sitting' (as there may be others next to Tono. Without 'yang' it still refers to the same person, but 'yang' provides more focus or direction.