Teeth actually bend at the microscopic level. The enamel is thin in the areas close to the gum line. As the teeth bend from excessive grinding forces, the enamel shears away from the tooth where it is thin, at the gum line. This exposes the underlying tooth structure, the dentin, which also stains over time. This is the discolouration in the first photograph.
Once the dentin is exposed, the abrasive nature of toothpaste can wear these areas even further. These areas can be sensitive to cold and sweets.
The teeth above where repaired very simply with dental composite resin material but the patients underlying grinding problem has to be addressed as well. This often requires an examination of the jaw joint and the occlusion, or the way the teeth fit together. The treatment may be as simple as a night guard to protect the teeth from grinding forces and an occlusal equilibration, which means to balance the bite so the chewing forces are more evenly and appropriately distributed among the teeth.