The water resistance ratings on watches confused the heck out of me when I first started getting interested in watches and shopping around, so I made this video primer. Basically:
30 meter/ 3 bar /3 atm - Splashes, rain, hand washing, maybe brief submersion. Not recommended for showering
50 meter / 5 bar / 5 atm - Light swimming probably ok, but not really designed for it.
100 meter /10 bar /10 atm - Fine for swimming, snorkeling, etc. Have taken these in the shower without steam penetrating the case. For most people a watch with this rating will cover all the situations they would reasonably subject a watch to.
200 meter / 20 bar / 20 atm - Recreational water sports, snorkeling, light diving.