Texels are similar to pixels from a picture except they are parts of a texture. They are sometimes interpreted as “puzzle pieces”. They are not the same thing as pixels because a texel is a container for the pixels. However, texel density (TD) is the amount of resolution on a mesh. Even if you have a high resolution monitor, the resolution of your mesh can be still made blurry if the TD is low. You want to keep all of the texels in uniform size so that the resolution on the mesh is the same resolution across the whole mesh.
The TD can be effected by some things such as:
- Mesh size
- UV shell size
- Texture resolution (texture map size)
MWorthy NextGen Blog 