Traffic light labelling is used by the manufacturers to provide an information on fat, saturated fats, sugar and salt content in foods. The labelling uses three colours: red, amber and green. Red products should be consumed less often and in smaller amounts and green indicates the healthier option. Amber colour-coding means that the product is neither high or low -- 'medium'.
Traffic-lights labelling for food (source)
Traffic-light labelling for drinks (source)
It is important to understand, that the information is provided either per 100 g/ml or per portion. The latter is used when the portion size is more than 100 g or 150 ml. Therefore, when you choose the sandwich in the supermarket, check what they mean by 'portion size'.
National Diet and Nutrition Survey in UK shows, that British people have increased intake of sugars, salt and saturated fats (read about it here). To follow the healthy diet, it is important to minimise the intake of these constituents. When you choose your sandwich, look for a healthier alternative and choose green or amber options.
The label also provides information on the energy content. It is written on the white background, but it still essential to follow the energy requirement recommendations (2000 calories for women and 2500 calories for men) and avoid exceeding them.
So, what does it all mean in practice? Remember, that one food cannot be classified as 100% healthy or unhealthy. It is the balance, that makes the diet healthy. For example, a tablespoon of the Sainsbury's peanut butter is labelled high both for fats and saturated fats. However, it accounts only for 6% of recommended maximum saturated fats intake and 14% of fat intake. A can of coke is labelled red for sugars and contains 39% of the recommended maximum sugar intake. These two foods will be detrimental for the diet if consumed in high amount every day. But nothing stops you from a little treat now and then!
If you want to follow a healthy diet, you can still afford to get the food products labelled with red, but you should balance it with healthier options (e.g. you can get a salty sandwich with bacon, mayo, cheese and get a low-fat, low-salt dinner later).
Are there foods you should avoid completely? If your aim to get a perfect balance in your diet, avoid the foods that are labelled red and contain more that 100% of the daily recommended intake:
Daily recommended intakes for salt, sugars, saturated fats and total fats (2000 calories diet):
Salt - no more than 6 g, sugars - no more than 25 g/5% of the total energy intake, total fats - no more than 78 g/35% of total energy intake, saturated fats - no more than 24 g/11% of total energy intake.
For more information on balanced diet head to the NHS pages