Protonilus Mensae is like a big scratching post on Mars. Just like how cats like to scratch on posts, Mars has big rocks that stick out of the ground like posts. And these rocks are called mesas. Protonilus Mensae is just one of many big scratching posts on Mars.
But why is it called Protonilus Mensae? Well, "Protonilus" means "first river" in Latin. And "Mensae" means "table" in Latin. So, Protonilus Mensae is a big scratching post that's kind of shaped like a table and it's in the region where the first river on Mars was discovered.
Basically, Protonilus Mensae is a really cool place on Mars that scientists are interested in because it may have clues about the planet's past and whether or not there was ever water on Mars. And that's important because it may help us understand more about the possibility of life on other planets.