https://teanmoon.blogspot.com/2019/01/weights.html
Sims bodies are made of a few different components. First there’s the mesh. This is the general shape of the sim made of vertices (verts) and polys. The mesh itself has lots of different data assigned to it including vertex paint, weight paint, and the uv-1 map. The other major component to a sim is the “skeleton” or rig. The rig is what controls animations and the rotation of different sections of the rig is what controls the poses a sim makes.
This shows the rig and sim base mesh (which is also sometimes referred to as the rig). The black dashed lines and dots show the location of the rig. Each dot is a different bone
Bones – These are the sections of sim’s body controlled by animation. They act much like bones do in the human body although much more simplified. For instance, short hair is assigned nearly exclusively to b__Head__ (The head bone) which means that any time the sim’s skull/head moves, the hair should move with it. Orangemittens of S4S has complied a list of all the bones (Joint List For Sims 4 Sim Skeletons )
Weights – Weights are what connects the sim mesh to the bones. They are values from 0.0-1.0 assigned to each vert for each bone. The end result is actually a matrix where you have three aspects: the vert location in x,y,z, the bone, and the weight value.
So for instance we could have a vert located at -0.0876, -0.0455, 1.507. This vertex is assigned to four bones on the base mesh: right shoulder twist, spine 2, right clavicle, and spine 1. Each of bones has a weight for that particular vert.
You can see the selected vertex circled in red. Its x,y,z location are listed at the top of the right panel, under Transform. Under Vertex Weights we can see the names of the four bones assigned to this vert. And next to each bone name is the weight assigned to that bone at this vert. In this case we see spine 1 has a weight of 0.235 at this vert.
The other important thing to note about the weight values is that they must all add up to 1.0 for each vert. When the values all add up to 1.0, we say that the weight values have been normalized.
Weight paint mode is a great tool for visualizing weights.
Here the bone b_spine1_ is selected (the cyan dot). The top mesh shows a visual representation for the weight values assigned to the top mesh.
What weight paint is really showing you is a visualization of the weight values at each vertex. Each vertex appears as a single color.
In weight paint mode dark blue represents 0.0 and red represents 1.0.
Here's a color breakdown: