The phenomenon is called punctate or streaked leukonychia. Contrary to popular myth, it does NOT mean a lack of calcium in the body.
Its appearance translates to damage to the keratinization of the distal nail matrix, which gives rise to a portion of the nail and which can be seen through the transparency of the nail plate in the form of that white crescent called the lunule.
The matrix can be affected for example by:
mechanical trauma (aggressive manicure, the habit of biting nails)
various eczema around the nail
The dots or lines migrate distally towards the end of the nail as the nail grows, later being removed when the nail is trimmed.
When you present yourself for a consultation with the dermatologist, it is good to remove any kind of nail polish in order to be able to provide visual access to them, because their changes can indicate certain systemic conditions.