A chain letter is like a letter from a friend, but instead of just reading it and enjoying it, your friend asks you to send this same letter to all your other friends. And then those friends do the same thing and send it to all of their friends. It's like a big game of telephone where everyone has to pass on the message.
The letter will usually say that if you don't send it on to other people, something bad will happen to you. But if you do send it on, something good will happen. This is called a superstition. It is not true.
Some chain letters will also ask you to send a small amount of money or a small trinket to the person at the top of the list. This is called a pyramid scheme. It is illegal.
Chain letters started a long time ago when people would write letters and send them through the mail. Now, most chain letters are sent through email and social media. But, just like before, they are usually a waste of time and not true.
So, it's best to just ignore chain letters and not participate in them.