It’s a ticket for use of the interstate, basically. Used to be a bit of paper you’d stick in your windscreen, now it’s electronic in a lot of places. Rather than a toll, which may be additionally applicable, it’s a standing fee valid for a set period.

They’re usually inexpensive. Most countries have clear and repeated signs telling you you need one, etc.

Slovenia - last I was there - don’t have a single sign on the way into the country. They don’t tell you at the toll post on the border.

What they do have is cops with binoculars sitting in the median flagging down every foreign plated car, and then issuing a fixed €200 fine (they’re ready with a card machine), and telling you you have to buy a vignette at the next gas station, which is where they’re sold.

Anyway. Wildly irregular across Europe. Useful site!