A tumbril is like a big wagon that people used to use a long, long time ago. It was made out of wood and had four big wheels, so it could easily roll over bumpy roads. People would use a tumbril to transport things like hay, grain, or even people!
But, unfortunately, tumbrils are most famous for something really sad. Back in olden times, people would use tumbrils to take prisoners to their punishment. This was usually a very bad punishment, like getting locked up in jail forever, being whipped, or even getting their head chopped off.
So, a tumbril is kind of like a wagon that was used a long time ago to carry things or carry people to their punishment. It's not really used anymore because we have better ways to transport things and, thankfully, we don't punish people like that anymore.