"Nesting" means that you look at your miter saw like this: See, or envision, the lower surface (where the wood rests) as a substitute for the surface of your ceiling. Hence, the fence (the back vertical part) can be envisioned as a representation of your wall. They obviously meet at 90 degrees... just like your ceiling and walls do.
"Nesting" cutting dictates that you are going to have to begin by placing your molding on the saw base upside down. In other words, the top edge of your molding (the edge that will go against the ceiling) is placed against the part of your saw you are pretending is the ceiling surface. That part is the base surface, the section with the slot the saw blade drops down into when cutting. The molding sits on its edge, front part toward you, on the base with that top edge of the molding downward.
The other edge of the molding tilts back and rests against the fence of the saw. Remember that we are calling the saw fence a substitute for the surface of the wall.
The molding rests, "nests", at an angle against these two surfaces just as it soon will be "nesting" when it is installed in your home. Notice how the back edges of your molding have small, flat surface edges running their full length? Look closely at those two edges when the molding is sitting on your saw as I have tried to describe. See how they probably are NOT flat against either the saw base plate nor the fence? Slide the tilted molding forward or back a little bit (toward you or away from you) till you can see how the two flat back edges soon fit flush and smooth against their respective surfaces.
When the angled, or 'tilted'. molding is sitting firmly and flatly against both surfaces at the same time, the molding is known as being properly "nested" . It is now sitting ("nested") at the same angle it will assume when installed up there above your head.
Do not be dismayed that it is upside down. This is how you want it for now. You will cut all of your pieces this way, always. Upside down.
Not only that, but you will also cut all your pieces "backward". (because your saw setup is the opposite of, or upside down to the actual position and orientation of your walls and ceiling) In other words, a piece that will fit on your ceiling on the LEFT of a corner as you are looking up at it up there, will be cut on the RIGHT side of your saw blade down here. And a right hand piece will, likewise, have to be cut from the LEFT side of the saw.
This is commonly referred to as the "upside down and backward" method (How about that?).... or "nested" cutting.
I have another post on this that explains this strange sounding method in more detail which I'll try to dig up.
In the meantime, think of this kind of molding cutting as you would experience trying to paint a picture while looking at your canvas through a mirror, and things might begin to make more sense. You're working in reverse, so you do have to alter your normal thinking a bit.
In case I didn't mention it, you never bother to use ANY of those confusing angle markers and settings when using the "nested" method. You leave the bevel tilt of your saw on ZERO the whole time. All you ever do is swing the blade to 45 degrees, either right or left, nothing more than that simple adjustment. (Well, I said 45..... sometimes your walls are not always built at a true 90 degrees, so there often IS some minor adjustment of a degree or two required for perfect fits.)