Hi Elly, it's confusing isn't it?
Basically the Xenical works like this:
You take your tablet and eat your meal. The tablet then 'clings' onto a third of the fat in that meal (whether it's a low fat or a high fat meal) and gets rid of it in your stools. The other two-thirds of the fat is absorbed by your body in the normal way.
So, say you eat a meal with 9g of fat, 3g of that fat will go straight through your body (without any side effects, because it's such a small amount) and 6g will be absorbed by your body.
Say you ate a meal with 60g of fat, 20g would go straight through your body (and you would probaby get some nasty side effects, because that's a lot of fat to come out in one go

) but 40g of fat would still be absorbed into your body, for you to burn off later (or not!

)
So using this example, it's best to stay with a lower fat meal, partly because you'll get the side effects if you don't, but partly because your body has still got to deal with the other two-thirds of that fat that the Xenical doesn't get rid of.
Does that make sense? Xenical is a helping hand really, but the more you can do for yourself (ie eating low fat/calorie meals and exercising) the better your weight loss will be.