They will pee themselves, and end up being completely dirty and will require a shower. If you let your sim’s bladder need get extremely low, they’ll end up peeing themselves and this is an embarrassing moment for any sim. This also works for food, the more you eat, the more you’ll have to go! The more your sim drinks (either drinks like milk from the fridge, or mixology drinks from a bar) the more often they’ll need to use the bathroom.
This is one of the most straightforward of the needs, as it just requires that you use the bathroom, as soon as possible! You can get your sim to use the bathroom at any point, not just when they really need to use the bathroom so it’s a great need to plan ahead for.