Here's the essence of it: it's a sun-activated serum containing Pro-vitamin D that you apply once daily or before sun exposure. You allow yourself at least 5 minutes of direct sun exposure after applying the serum and refrain from applying any other products until your sun exposure is complete. This serum accelerates your skin's vitamin D production, reducing the time needed in the sun.
There are many things that people mean when they say they have sensitive skin. Typically people are describing skin which has a lower tolerance to specific skincare ingredients and that they experience redness, swelling, itching, burning or stinging sensations. Causes commonly include acne, rosacea, a weak skin barrier or contact dermatitis from allergens.