Textures are represented by texels in the same way that pictures are represented by pixels. They are the smallest “puzzle piece” of a texture. They are not the same thing as pixels because a texel is a container for the pixels. So naturally, texel density is the amount of texture resolution on a mesh.
To make it easier to understand, texels contain pixels within a 3D model. Texel Density is the consistency of the pixels within a mesh.
Daniel Porter 