Sorry, sorry - disappeared there for a while.
GI - as agedknees says, is Glycaemic index - it relates to the speed at which the food causes blood sugar to rise. I.e. how fast the sugar part is absorbed. There are different scales, some measured to pure sugar and others to white bread (which is very high anyway) but there are some silly results on it.
For example: carrots have a high GI on their own, because they are quite sweet; so as an extra refinement to this scale, they introduced GL = glycaemic load.
This takes the GI of a food but includes the actual sugar content of the food as well - carrots may be high GI but they are low GL and therefore are good to eat.
All fat is not bad - do not get dragged down the low fat route as it will impede your ability to lose weight sensibly. Especially as a lot of low fat food is very high in sugar/carbohydrates and is therefore very high GI and often high GL as well.
There are lots of books out there on GI/GL diets - I would look for one that uses both scales.
But, if you don't want to go for a book, then the best thing to think about is eating everything in its most natural state - as unrefined as possible. Whole grains rather than white flour; whole veg and fruit rather than juices; natural fats rather than chemically altered ones (olive oil better than most).
Eating fat with a carbohydrate-heavy meal will also slow down the absorption of the sugars from the carbs; so for e.g. having a baked potato WITH mayo/cheese/butter/oil/any other fatty food will have a lower GI than having a baked potato on its own or with no fat.
Skinny Cow ice cream has a much higher GI than ordinary ice cream as there is next to no fat to reduce the sugar hit. (I hate it for that reason, it's bloody misleading. Just like those Go Ahead biscuits, which are full of bloody sugar)
HTH - happy to answer any other questions.