You can show poop, say cuss words like 'bitch' 'piss,' 'ass' and 'shit,' (as long as it's not too often) and show slightly more violent or sexually themed material.
Go overboard, show a nipple or any other nudity, say fuck, cunt or tits, or get too gory, and it's R Rating Country.
Famously, the movie 'Clerks' was slapped with an R rating purely for discussions about sex acts. It doesn't have any nudity or violence in it, per se.
Really, the MPAA are full of shit. But they do have a few rules which are strictly enforced.