"Der Hund isst eine Biene."
ist and isst are homophones (they sound exactly the same). In the "real" world, you would know which was being said by context. In Duo (when given the audio) most of the time both are accepted because either sentence would be possible. In this sentence, I guess you could say "the dog is a bee" if you dressed your dog up in a bee outfit, but 99% of the time it should be "the dog eats the bee". Hope that helps!