The previous post was a more generalised guide to weight loss. If you are relatively slim already, or those tips aren't working then you may need to calorie restrict. Calories are the most commonly used units of energy in relation to humans, joules are the better but less used alternative. Your body needs energy for its physiological functions, not enough energy long term = you die. You get this energy from food and drink. If you eat more calories than your body needs, you store it, primarily as fat. If your body doesn't get enough calories, your body uses its own energy stores (primarily fat). Calorie restricting is simply reducing your calorie intake, below what your body requires. This forces your body to use its fat stores (though there may be some lean mass lost).
For some terminology and referencing points 1 Cal = 1000 cal or 1k cal. 1 cal = 4.2 joules. 1 kcal is the energy required to heat 1 kilogram of water, by 1 degrees Celsius. 100 grams of chicken leg = ~ 240 kcal, 100 grams of broccoli = ~34 kcal, 100 grams of butter = ~717 kcal.
When calorie restricting the first thing to do is work out how many calories you burn in a day, which is commonly referred to as basic metabolic rate (BMR) or total daily energy expentiture (TDEE). TDEE is a more accurate estimate as it takes into account the physical activity you do during the day. There are a variety of ways of working out your BMR or TDEE. You can figure it out manually, using formulas such as in this BodyBuilding.com thread. Alternatively you can use one of the many free applications on the internet, on sites such as FitDay. Its important to note that whatever figure you come up with, is an ESTIMATE only and should be used as such. If you want a more accurate figure then you'll need to go a physiology lab or medical facility.