Societal means 'to do with society as a whole' — with the big shared structures people live inside: laws, customs, schools, families, money, what is expected of men and women. It is a formal, written word. You meet it in reports, research papers, news analysis, and company statements, in phrases like 'societal change', 'societal norms', 'societal pressure', 'societal cost'. Here is the useful part: English already has 'social', so why does societal exist? Because 'social' does too many jobs. 'Social' can mean 'to do with friends and parties' (a social evening, social media, a social person) as well as 'to do with society'. Societal cannot mean the party one at all — it only ever points at society, the whole system. So a writer picks societal to be unmistakable, and to sound weightier. Compare: 'social problems' could be one person's trouble making friends; 'societal problems' can only mean problems in how the whole society is built. There is a scale to it, too. Societal usually means large and slow — things at the level of a whole country or culture, not one office or one family. Some careful writers dislike the word and think 'social' would do; you will still see it everywhere.