Frost or sometimes referred to as white or hoarfrost happens when the outside temperature drops to 32 degrees or under and forms ice crystals on plant leaves, injuring and sometime kills plants. When skies are clear/calm and afternoon temperatures drop that is usually the perfect time for frost to occur.
If frost is predicted- cover your plants! This helps lock in the soil heat and moisture to protect them against strong winds. You can cover the plants with newspapers, baskets, tarps, straw, and other materials. Make sure you cover the whole plant before sunset to trap in any remaining heat and lightly anchor the coverings so they do not blow away.
When frost is predicted, water your plants that day. As the water freezes it releases heat and protects your plants even though they are covered in ice. You may even leave your sprinklers on all to ensure the plants are being properly coated with water for protection. However, your sprinklers need to be running continuously as long as the temperatures remain below freezing to prevent any water damage.