This is one of those questions where if you ask 10 different people, you'll get 10 different answers!
There isn't a simple answer because art itself is hard to define. The word art can be used to describe any skill that produces something. Every civilization and culture has forms of artistic expression. So there are lots of categories of arts, and even more theories about what makes an artwork good. And what does "good" mean, anyway? Nice to look at? Practical and useful? Wildly imaginative or super realistic? You get the idea...
An artist uses skill and imagination to create something that makes people respond to it. Art can be a statement or an exploration of thoughts and feelings. Since there's no formula to calculate its meaning and value, how we judge a work of art is a constant matter of debate. Humans have been thinking and writing books and arguing about this topic throughout history. Depending on what approach you take, you could find different meanings in an artwork.
Is there a famous painting or sculpture that makes you think "I don't get it"? Here are some questions you can ask yourself: