What's yellow and can't swim?
A bus full of toddlers.
Okay, so basically, the joke is that a bus is yellow, and toddlers are small humans, and humans can't swim if they don't know how. You probably don't know how. I can swim. I am a bear. Bears are naturally good swimmers, unlike you. Did you know that bears can hold their breath for a long time? This is a useful skill for catching fish, which is another thing you're probably bad at.