Horizontal ridges on nails, often called Beau's lines, are often the sign of a health problem, according to MedlinePlus. These ridges occur after an illness or injury to the nail and sometimes result from chemotherapy or poor nutrition.
Not to be confused with the more common vertical ridges that are associated with aging, horizontal ridges indicate a systemic health problem, according to EmpowHER. Nails stop growing when an individual is very sick or experiencing a lot of stress. Once the nail starts growing again, a horizontal indentation is often the result. An horizontal indentation on a single nail also signals the presence of a wart or cyst at the nail's base.