Hey there!
Fertile means very suitable for farming - in other words, very rich and full of nutrients plants and food need to grow. When you say soil is fertile, often that means it's capable of producing massive food surpluses due to the fact that it can grow so much.
It's a region because it has the two rivers, the Tigris and the Euphrates around it. Unlike other regions around the area, the soil was unnaturally fertile, and that's why it takes up it's own region because it's not the same anywhere else.
It's a physical region. Humans did not create it, and the soil is part of what makes it physical. The settlers used the soil, but didn't make it.
Hope this helps!
