"Shower thoughts" are the random little realizations you have when you're in the shower (or other places, but the shower thing is what stuck) and you're just letting your mind wander without any particular focus. While it's definitely possible to have ACTUAL breakthroughs that help advance the knowledge of mankind, most people's version of shower thoughts could definitely also be thought of as stoner thoughts - stupid stuff that makes you go "Woah!" but isn't actually worth anything.