What is veining?

Veining is the process of making very thin straight or curved cuts in the surface of wood or other materials.

The bottom of the grooves are usually rounded, like veins.

Isn't that the same as engraving?

No, engraving can be done with a router bit, but it involves removing much less material by scratching or etching the surface, rather than making the deeper cuts associated with veining.

Veining is most commonly used for lettering and numbering applications, with the aid of a guide bush and appropriate template.

It is usually done with a very thin V-groove or round nose router bit. There are also panel veining cutters available.