See below. A compromised tooth (fracture, failing old filling) may need to be covered and protected by a crown. First, there must be enuf healthy tooth as a foundation. It may be necessary to do a "core buildup". Then the tooth is reduced in all dimensions (ground down if you must), a mold (impression) is taken, and then a crown is custom fabricated to recreate the tooth that was once there.
A little grinding. Teeth are prepared for a crown by trimming the tooth in various ways. Teeth can be restored with all non precious metal, semi-precious or more expensive metals. Even titanium nowadays. They can also be restored with ceramic crowns. It depends on your pocketbook, your preference of material and the location in your mouth. Want a gold front tooth? It can be done.