To put it simply, an ephemeral tattoo has the same principle as a decal.
It is composed of specific inks which remain on the skin between 3 to 7 days.
These inks are harmless because they are governed by the same regulations as in cosmetics.
You just have to put the drawing against your skin, wet it for 30 seconds and the transfer is done directly... a real breeze.
Waterproof, it allows you to shower / bathe quietly. However, be careful with hot baths, this could damage it.
Here is a demo video: