The idea is that a deciduous plant goes dormant in the cold winter to protect itself from the cold. The plant needs to stay dormant while the weather is freezing and then know how soon after it gets above freezing it can safely start growing. It must do it late enough so it doesn't get frozen back by a late frost but early enough so it can get a full season of growth and fruiting in before it must go dormant for the next year. The plant has a process, refined over millennia of evolution that tells it when to start growing in the spring and that process accounts for the amount of above freezing temperature (chilling hours) it needs.

Of course when we play with mother nature and grow plants in climates where they are not native, we run into lots of problems and this is one of them.

CHILL REQUIREMENTS
TYPE OF FRUIT CHILL HOURS
Almond 500-600
Apple 400-1000 (Low chill varieties are less)
Apricot 500-600
Japanese Pear 400-500
Blackberry 200-500
Blueberry (Northern) 800
Chestnut 400-500
Cherry 700-800
Citrus 0
Currant 800-1000
European Pear 600-800
European plum 800-900
Fig 100-200
Filbert 800
Gooseberry 800-1000
Grape 100+
Japanese Plum 300-500
Kiwi 600-800
Mulberry 400
Peach 600-800
Persimmon 200-400
Plum Cot 400
Pomegranate 100-200
Quince 300-500
Strawberry 200-300
Raspberries 700-800
Walnut 600-700