These stamps commemorate the Eclipse occurring right across the US on Aug 21 2017
You can buy a set of the eclipse stamps at the US Post Office. And for another 25 cents you can purchase the special envelope for them.
They are also available on amazon.com here:
You press on a stamp with your finger or thumb and the heat from your finger will....
And the moon appears.
And in a short period of time, as the stamp goes back to room temperature the moon will disappear again into the blackness of eclipse.
Want to learn more about the eclipse? NASA has a neat map that you can download. It will show you how much of the eclipse you can see depending on where you live. Download it here: NASA PDF on the eclipse