Food is split into three groups; Free food, Healthy Extras and Syns. The majority of your food for the day should come from the free food group
You can eat as much as you like of free foods (eggs, veg, fruit, lean meat, fish, pasta, rice, pulses - with a few exceptions), plus a few other things I've probably forgotten.
You get one healthy extra A and B choice per day (HeA and HeB) which are limited portions of dairy (HeA) and bread/cereal etc (HeB).
Anything not in any of these catergories is assigned a 'syn' value. You can have up to 15 syns per day.
Thats the VERY basics. There are also things like speed foods and the Extra Easy SP plan but I wouldn't worry too much about those at first.