According to MedicineNet, a precursor to a vitamin is an inactive substance that later converts to a vitamin. Any chemical that eventually transforms into another is known as a precursor.
According to Florida State University, beta-carotene is a precursor to vitamin A. Enzymes in most animals, including humans, convert beta-carotene into vitamin A. This allows people to get vitamin A by eating plant-based foods. Beta-carotene is present in asparagus, broccoli, carrots, pumpkins and spinach, among other plant-based foods, but vitamin A itself is present only in food products derived from animals. Vitamin A plays an important role in encouraging bone growth, maintaining vision and maintaining healthy skin and hair.