Think about it this way. The "mesh" is essentially what gives any game object its underlying shape. Imagine trying to simulate a realistic looking sphere. Your ability to make that sphere more accurate looking is ultimately dependent on the number of "nodes" (or points that can be manipulated into position). See the picture below for a visual explanation of this.
As you increase the number of nodes, you can come closer and closer to simulating the 3-dimensional curvature needed to generate a more realistic looking sphere. The same principle applies with computer gaming. If you increase the number of nodes for any given object, you increase your ability to make the underlying mesh form to whatever real world object you are trying to simulate.
In addition to the "mesh", you also have the "skin". Think of the "skin" as a 2-dimensional picture that is wrapped over and around the mesh in order to simulate the material and look of that object.
Jigo, you're old enough to remember VCR technology. VHS tapes sent a signal to your television that was 352 x 240 pixels. Nowadays we have Ultra HD (also known as "4K") Blu-ray players that send a signal to your TV that is 3840 x 2160 pixels. Imagine comparing the image quality of a VCR and an Ultra HD Blu-ray player displaying the same material on identical 60-inch 4K televisions. There would be a monumental difference in the quality of the appearance of the two playbacks despite using the same televisions.
It's the exact same principle with computer graphics. The higher the resolution (the more pixels) of the "skin", the more detailed and realistic looking the picture is that is wrapped around the mesh of whatever object it is you are trying to simulate.
So essentially what I've done is to switch out a lot of the standard Skyrim meshes and skins for ones with more nodes and higher resolution. That allows me to achieve much more realistic looking graphics in the game than I would have if I only had Skyrim in its original configuration.
But I've done more than just that. I've also modified the way that light and shadow are treated within the game. In real life, light and shadow play an enormous part in how we perceive 3-dimensionality. Most of the aspects of their behavior are extremely subtle. But it becomes easily detectable to your eyes when one or more of these parameters isn't quite right or its missing altogether. Your eyes take visual cues from things like reflection, hue, saturation, bloom, ambient occlusion, subsurface scattering, chromatic aberration, focus, etc., and it allows your brain to make determinations based on what your eyes are seeing. By more accurately simulating in Skyrim the way that light and shadow (and your eyes) behave in the real world, it drastically improves the look of my game.
There is even more that I've done to make my game more realistic looking, but those are the basics. By modifying the game in these ways, it allows me to go from what "out of the box" Skyrim looks like...
... to this: