During the day, soil collects heat, whilst overnight soil loses heat. Covers trap the warmth of the soil, keeping warmer air down close around the plant, thus preventing the rapid surface cooling conducive to frost formation. Additionally then, the cover, rather than the plant, becomes the surface on which the moisture freezes.
It’s common practice to cover frost-sensitive plants, using anything from old baskets and boxes, to bed-sheets, curtains, vinyl and plastic, shade-cloth and hessian (burlap) cloth.
A commercially available cover is Frost-Cloth or Horticultural Fleece, a light, breathable fabric made from spun-bonded polypropylene or poly-ethylene.