Everyone has a certain caloric requirement per day. If you eat less than this number, you will lose weight. If you eat more than this number, you will gain weight. But if you eat just about equal to that number of calories, you will maintain the weight. Most people who are overweight, put that weight on during a certain period of time in their life, but then level off and continue to maintain that weight. The hardest part is losing the weight, because then you actually have to eat less than your body wants you to. It is understandable that you lost weight when you ate less the week before, but you aren't gaining the weight back because you are now eating enough to simply maintain your daily requirement.

That is why you sometimes hear people say "How can a 140lb person eat the same amount as a 200lb person, but not gain weight? Well, they are both maintaining their weights by meeting their daily caloric requirements. The reason the 200lb person weighs more is because he may have regularly eaten beyond his daily caloric requirement over time in the past.

Of course, this is a simplified summary. There are other factors involved. But I think you get the point :)