It would help to know if these indentations are longitudinal (running from cuticle to free edge) or horizontal (from side to side) because they have different causes.
Possible causes of horizontal ridges are beaux lines and habit tic disorder. Horizontal ridges can also occur from abnormalities of the cuticle area associated with eczema or paronychia. Longitudinal ridges have many causes as well. Wide grooves can be caused by a space-occupying lesion in the nail fold area just overlying the matrix (root) of the nail. These grooves are as wide as the lesion — which is usually a myxoid cyst, but could be a wart , fibroma, or something else. Also we see grooves associated with ridges due to aging and lichen planus. A dermatologist would be able to make a definitive diagnosis. — Dr. Phoebe Rich