'Hella' is Northern Californian English for 'very' or 'a lot of'. 'Hella good' = 'really good'; 'hella people' = 'lots of people'. Originated in the Bay Area in the 1980s and spread nationally and globally. If your teen uses it, they are using American intensifier slang. Harmless and conversational. Word of caution: in some communities the religiously-objectionable origin (rhyming with 'hell of a') makes older speakers prefer alternatives.