If you mean the spraying... it's kinda weird. Water actually creates latent heat as it freezes. I Googled it and found a more elaborate explanation. The second one is what I've often heard.
"What the orange growers are relying on is a thermodynamic property (in this case, of water) called latent heat of fusion. Basically, since water can exist in its solid and liquid form simultaneously ONLY at 32° F, it will stay at that temperature until it is all frozen. So for a short-lived cold snap, this strategy can protect one's orange trees by having the sprayed-on water freeze essentially instead of the orange. But if it stays below freezing for an extended period, the oranges will freeze, too."
"Water keeps trees from freezing due to the fact is releases heat when it freezes, each gram of water that is frozen releases 80 calories of heat. That heat raises the temperature of the tree, the air around the tree and the soil, preventing ice formation in the tissues of the tree."