# What forces of stress really represent?

I'm studying fluid mechanics and the book says the following:

For any continuum, forces acting on a piece of material are of two types. First there are forces of stress, whereby the piece of material is acted on by forces across its surface by the rest of the continuum.

Now, I can't get the significance of this. Since he says "forces across its surface" it seems that the piece of the material must be three dimensional. Is this right? So, forces of stress are always exterted on three dimensional pieces of a material?

What I mean by this question is: suppose $D\subset \mathbb{R}^3$ is a three dimensional region filled with a fluid, then forces of stress are always exerted on three dimensional subsets of $D$?

Also, what they represent? As I understand, they represent the force that the rest of the fluid exerts on that piece of it, but why they exert such forces? Is it some sort of force exerted at molecular level that has this macroscopic effects?