I don’t know if this is a good analogy, but this is how it was explained to me: I want to send things to people, so I give anyone who asks a key. I keep a bunch of lockboxes that can be opened by that key. When I send them stuff, I lock it up in that box. They know it’s from me if the key works.

I also have a bunch of free boxes in a pile, anyone can grab one, but only I have the key to those. They want to send me stuff? Only I can get into it.