This refers to cloth or polyethylene fabric that is used to literally cover a row of vegetables. Lightweight row cover can be used in the spring to protect seedlings from insects. The cover is placed over the whole planted area and its edges are tucked into or stapled into the soil. This is a very effective way of protecting carrots from the carrot rust fly – as just one example.
Heavyweight row cover is thicker, and is meant to be used as an insulating blanket in the fall or early spring. This will keep frost from damaging vegetables. Heavyweight row cover has several other uses, including wrapping tender perennials over winter, creating windbreaks, and so on.