A 'side quest' in video games is an optional mission separate from the main storyline. The term has been borrowed into youth speech to describe an unplanned, often funny detour in everyday life. 'I went on a side quest to find iced coffee' = 'I took an unplanned detour for coffee'. If your teen describes their day in terms of side quests, they are using gaming metaphor for ordinary errands and adventures. Harmless and playful.