A project of the Electronic Frontier Foundation.
Each person in a conversation has a way of creating two keys. One is their private key, which they keep to themselves and never let anyone else know. The other is a public key, which they hand out to anyone who wants to communicate with them. It doesn’t matter who can see the public key. You can put it online where everyone can see it.
The “keys” themselves are, at heart, actually very large numbers, with certain mathematical properties. The public key and private key are connected. If you encode something using the public key, then someone else can decode it with its matching private key.