Imagine you have a big sand pit in your backyard, and you dig a hole that goes really deep. As you keep digging, you might hit a point where the ground underneath is harder than sand, maybe it's rock or clay.
Well, imagine if that hard ground that you hit was a big crack in the earth's crust. This crack might go down really far, and it's called a fossa.
Fossae can be found in different places, like on the ocean floor or on land where there are mountains or tectonic plates moving around.
Sometimes, when two plates crash into each other, it can make a fossa. Just like when you push your hands together really hard, it might make wrinkles or folds, the same thing can happen to the earth's crust.
So, a fossa is basically a deep crack in the earth's crust that can be formed by plates moving or colliding into each other.