What is the white dash on my nails? Does it mean I have lack of calcium? It appears generally when I don't eat well. Thank u.

Trauma. Usually from trauma and having your nails hit against things. Maybe playing too many video games when you aren't eating too well! Haha. Honestly, not worried if they are short little white lines that grow out by about 3 mm per month.
Not calcium. Due to injury ---- usually minor--- to cuticle. Not from diet. Avoid any pushing or cutting or picking or biting the cuticles. A nail is not a living tissue, it cannot repair itself. It takes about 6 months to grow out a new nail.