In business terms, value added refers to the extras or features that a company adds to a product before selling it, to make it worth more to the consumer. For instance, if I sell vacuum cleaners, I could buy a lot of these from a supplier and add on a 5 year guarantee before I sell them on to my customers. The guarantee makes the product worth more than it would be if you bought it directly from the supplier.
Value added tax or VAT is a tax that has to be paid on most business transactions. Businesses have to pay this when they sell a product, for example to a shop, and this amount is added on to the purchase price. You can learn more about VAT here.
Sometimes the term value added is also used in a non-business sense. In this case it usually means something that is added by an institution or organisation. For example, with school league tables, which are usually based on things like exam results, you can include a value added component which looks at what the school has actually contributed: Eg, if the school is in a very poor area where fewer children can expect good exam results, you can take that into account when calculating how much difference the school has made to the pupils' progress.