I really like the example image in this article. The image asks the computer if there is a cat or a dog in the picture. As humans we can question the question being asked and reply with there is both a cat and a dog.
Computers rarely process problems in that way. It is difficult for them to realise they can answer the question in such a way without them being told the answer can be neither, a cat, a dog or both.
If Google can make a break through in this kind of AI it will greatly advance the way that computers are used in daily life.
A four-year-old child learns about the world through their senses so they know that cows don't fly without being told this. Computers need to understand some obvious things about the world so we want to build a common-sense database.