“Ποιο” in Greek means “which” .
Ποιο θέλεις, το άσπρο ή το μαύρο; = Which one do you want, the white or the blue?
Δεν ξέρω ποιο είναι το σωστό. = I don't know which is the correct.
Sometimes “ποιο” is used as “what”, however not very often.
Ποιο είναι το όνομά σου; = What is your name?
The most common translation for “what” is “τι” and not “ποιο”.
What do you mean? = Τι εννοείς;
What is this? = Τι είναι αυτό;