η βιντεοκλήση is used as a generic here -- the speaker is asking the listener to define or explain the concept of video calling or of videotelephony. A bit like η βιολογία είναι ενδιαφέρουσα for "biology is interesting".
But in English, we can't say "What is video call?" because "video call" isn't used as an abstract uncountable noun for the concept in the way that, say, "biology" or "videotelephony" is. So if you use "video call", you'd have to say "What is a video call?" even if you mean it generically.