This is the 103rd most frequent Croatian word.
što
"What" or "that" (depending on context).
Here, 'što' is used as an interrogative pronoun meaning 'what' in English, used to inquire information.
Što je to?
What is that?
Here, 'što' is used similarly as previously, introducing a clause that complements the predicate's assertion of uncertainty.
Nisam siguran što se dogodilo.
I am not sure what happened.
In this example, 'što' serves as a subordinating conjunction, introducing a noun clause explaining the content of not knowing—'he doesn't know what'.