This is the 354th most frequent Finnish word.
tosi
True; real; very.
In this sentence, 'tosi' is used as an adverb meaning 'really' to intensify the adjective 'mielenkiintoinen' (interesting).
Tämä on tosi mielenkiintoinen kirja.
This is a really interesting book.
Here, 'tosi' functions as an adjective and means 'true,' referring to the authenticity of the story.
Onko tuo tarina tosi?
Is that story true?
In this context, 'tosi' is used as an adverb meaning 'very' to emphasize the adjective 'vahvat' (strong).