I often wondered this myself, I'm new-ish to Slimming World, so I asked my consultant about it yesterday and got a leaflet.
In a nutshell, it's making sure you fill 2/3 of your plate with superfree foods and 1/3 with free foods - it's like following EE, but the ratio of free to superfree is reversed.
Here's a list of the superfree foods, hope this helps:
Amla (Indian gooseberry)
Apples
Apricots
Babaco
Bananas
Bilberries
Blackberries
Blackcurrants
Blueberries
Boysenberries
Carambola (star fruit)
Cashew fruit (not cashew nut)
Cherries
Clementines
Cranberries
Custard apple (cherimoya)
Damsons
Elderberries
Feijoa
Figs
Gooseberries
Grapefruit
Grapes
Greengages
Grenadilla
Guava
Jambu fruit
Jujube (Chinese date)
Kiwano (horned melon)
Kiwi fruit
Kumquats
Lemons
Limes
Loganberries
Loquats
Lychees
Mammie apple
Mandarins
Mango
Mangosteen
Medlars
Melon, all varieties
except watermelon
Mulberries
Nectarines
Oranges
Ortaniques (honey tangerines)
Papaya (paw paw)
Passion fruit
Peaches
Pears
Persimmon (sharon fruit)
Physalis
Pineapple
Plums
Pomegranates
Pomelo
Prickly pears
Rambutan
Raspberries
Redcurrants
Rhubarb
Sapodilla
Satsumas
Strawberries
Tamarillos
Tangerines
Tayberries
Ugli fruit
Watermelon
Whitecurrants
Yellow plums