A "priceless" thing is something that is extremely valuable. For example, "priceless" jewels are so valuable that you almost can't buy them for any amount of money.
When it comes to people, we talk about a "priceless" reaction when someone reacts in an extremely funny way to something. You can imagine that people use the word "priceless" to describe a funny reaction because they get such high entertainment value from seeing it. It's the kind of entertainment that you can't buy, no matter how much money you try to spend.
Here are some examples of "priceless" reactions:
- You see a friend who's reading a book on a bench. When you walk up behind him and put your hand on his shoulder, he gets extremely frightened and drops his book on the ground. You both laughed about it afterwards.
- Your high school English teacher was really strict and mean. One day you noticed a mistake that he made on a test and pointed it out in front of the class. He got really angry, and his face turned red. You thought it was funny and joked about it later with your friends.