This is the 249th most frequent Czech word.
vlastně
"Actually" or "in fact."
Here, 'vlastně' functions as an adverb emphasizing the information that the speaker already knew.
Vlastně jsem to už věděl.
Actually, I already knew it.
In this question, 'vlastně' adds emphasis to the speaker's curiosity about the true intention of the other person's words.
Co vlastně chceš říct?
What do you actually want to say?
In this statement, 'vlastně' denotes a realization or clarification about the importance of the matter being discussed.