It uses a word that has two different meanings, or two words that sound alike, to make a joke.
Think of it like this:
* Wordplay: A pun is a way of playing with words to be funny.
* Double meaning: The pun makes you think about the word in two different ways at once.
Example:
"I'm reading a book about anti-gravity. It's impossible to put down!"
* "Put down" can mean to stop reading a book, but it also means to place something on the ground.
* This makes the joke funny because the book is literally impossible to put down because of the subject matter.
So, a pun is a joke that makes you chuckle because it uses words in a clever and surprising way!