There are a lot of subjects in school that cover a lot of information. But most of it boils down to a few skills, which you can then apply at whatever subjects you wish to or find a need to, the most important of which is critical thinking. No one, no matter how much school they take, will know everything about everything. In fact, the further up you go in education, the more specific the work you study becomes. The better you want to know something, the narrower the subject upon which you can set your sights.
I mentioned the encyclopedia and wikipedia because they are tools. When reading books, news articles, and more, there will be unfamiliar material. Sometimes it can be figured out from context clues, sometimes you might need to look something up. And you can. You have that at your fingertips. It will not tell you everything, but you can look up what you want, and if you determine you want to know more, you can look into doing that too.
But you should certainly not feel stupid. Not everyone understands. Some people are better at hiding that. But here, in this example, two people who take interest in art and art history exchanged a comment apiece. Since Baelfire loves art and mentioned a college level art history class and since I am teaching an art history class, admittedly at the high school level, it's not surprising it has references you didn't understand. So very specifically, here, you absolutely shouldn't feel stupid. If anything, I should have explained more accessibly what I was speaking about.
I want what I teach to be accessible and of interest to those I'm teaching. That's why I made this post. EPS class sizes are small enough to allow some liberties in electives like Art History.