Okay so there's these bulletin boards right.

These boards are all over the place. Some are at school some are outside of people's houses some you have to pay in order to post something on it (to thumb tack a card on it). You and your friends used to always only post to one of these boards. This allowed you to share messages with each other in a way that they could check at any time and you didn't have to see each other in person to share information with each other.

Then, one day, the owner of that board said your friend isn't allowed to post on the board anymore because they posted dick drawings on it. You think pictures of dick drawings are funny and would have loved to see your friends humor but now you can't use that board to share with each other anymore.

So with nostr, you post the same message on many boards spread all over the place and maybe some of them don't allow dick drawings, but some of them do. So in your mailbox you get all the cards posted to all the boards all in one place and the cards are signed by the poster so you know it came from them.

Then when you want to post, you post your card to all the boards that you like to use and when your friend wants to see them, they get all the cards from all the boards in one place just like you.