Was soll das heissen? : What's the meaning of this/that? (I've also heard it as, "Was soll das?" and akin to "what do you think you're doing?" or "What's THAT supposed to be?" in tone and intent.)
zB: Was soll das heissen? Warum bist du nicht in der Schule?
auf Englisch: What's the meaning of this? Why aren't you in school?
(from the guide to German Idioms by JP Lupson)