This is the 162nd most frequent Russian word.
что
"What" or "that" (depending on context).
The word 'что' is used as an interrogative pronoun meaning 'what' in a question.
Что случилось?
What happened?
The word 'что' acts as a conjunction meaning 'that', linking the clauses.
Я думаю, что это хорошая идея.
I think that this is a good idea.
The word 'что' forms the compound pronoun 'что-то', meaning 'something'.