This mesh is intended to be used to create a wooden tabletop surface. However, the wood grain applied to the mesh (using default Roblox materials) does not seem to correctly display near the edges of the mesh. How do I fix this?
That happens when the mesh is stretched. A single pixel color is used for a complete line.
Blender? You need a wood PBR material of your own.
Go to the UV mapping tab and try “smart UV mapping” on the right side. On the left side, select and rotate individual sides so they all go the right direction. Scale all polys until the wood looks right (the larger the squares, the smaller the grain).
Those are the basics. You can watch videos to learn tricks.