This is the 573rd most frequent Czech word.
otázku
"Otázku" means "question" in English.
The word 'otázku' here stands as a direct object in accusative form of 'otázka', meaning 'question'.
Mám na tebe otázku.
I have a question for you.
'Otázku' is the accusative form of the noun, used as the direct object of the verb 'položil' (asked).
Položil jsem otázku učitelce.
I asked the teacher a question.
Here, 'otázku' is incorrectly used; the preposition 'nad' requires the instrumental case, so 'otázkou' would be correct.