The basics of it

There is one rule in the quest of weight loss, just like there is one rule in the quest to weight gain. It is simply this: Calories in VS Calories out.
All this means is, I must burn more calories throughout the day than what I eat to lose weight
I must burn less calories throughout the day than what I eat to gain weight.
Why? Because your body is like an energy bank... It takes calories to function just like a light bulb functions on units of joules. The amazing thing about the body though, is that if we take in too many calories by eating or drinking too much, then we store it as fat! Well this is wonderful news for us if we were going through famine! It is like saving up money for a vacation or retirement, or stocking the refrigerator with bottles of water and the pantry with canned goods. When we eat more than the body burns it can do nothing else more with it than to store it for later use. Now if you are on a quest to build muscle and put on some size, this method of eating more than you burn is very relevant to you because the extra calories are used to rebuild the muscle that you broke down during your workout. In this case, when done right, it works very well... but when your quest is to lose the extra storage, you don't need to keep stocking the shelves if you know what I mean. Use what you have and reduce your storage slowly.
Think about it like this, say you were in charge of setting the spending budget for your family. You notice that money is being spent frivolously and you know that if it continues to be spent in this manner, you will soon have problems. One way to get the budget back under control is to organize the spending into different categories. Then you might set "limits" on each category starting with an amount that everyone can agree on. When this "limit" is set in place, your family member, if they are in compliance, will know how much they can spend on a particular item in a specified period of time. For example, if you give you daughter a credit card and tell her to go shopping without specifying a limit, you might not know what was put on credit until you look online at the statement. If you give her a $100 bill and send her on her way, you know exactly what she has to spend.
If you know how many calories your body burns in one day, you can know how many calories you need to eat in order to lose, gain, or maintain your weight. It is that simple. Just as a budget has multiple facets, a calorie structure does, too. This we will get into later. For now, consider the concept that your in order to be in control of your weight, you must grasp the fact that it is all about Calories in VS Calories out.
Consider this and take a look at what calories you might be taking in and what you can do to offset any unwanted gains or losses with it.