'Arvo' is Australian English for 'afternoon'. 'See you this arvo' just means 'see you this afternoon'. It is a textbook example of Australian -o shortening, the same suffix that gives you 'servo' (service station), 'bottle-o' (bottle shop) and 'reggo' (registration). If your teen uses it, they have absorbed Australian slang from TV, YouTube or Australian friends. Friendly, casual, harmless.