Designer foods are also known as GM crops, or genetically-manufactured goods. At times they are also referred to as "functional foods".
These foods are genetically-manipulated with the purpose of adding or changing the nutrients and other properties of the original product. The rationale behind this process is that food can be made healthier, less fattening, or (in the case of crops) able to withstand the attacks of insects, as well as changes in weather. Hence, a stronger and more resistant type of crop may grow at a higher rate, producing more food for consumers.
However, those who oppose the production of functional foods claim that the genetic changes in these foods are counterproductive to human health. They also believe that the process is altogether anti-ethical. Attached to the answer is a link for more information.
Designer foods are foods created with the use of techniques like cross-breeding, genetic engineering, induced genetic mutation, etc. which are meant to enhance or reduce the presence of particular nutrients or modify them.
Designer foods can be created which have excessive amounts of nutrients which are considered very beneficial for health. Scientists for example have identified several nutrients like vitamin C, E, beta carotene and other phytochemicals that are very effective in reducing the occurrence of ailments which affect the heart; cancers and cataract and several foods have been created that have large amounts of these.
By increasing the number of anti-oxidants it is possible to help foods increase life span and keep people healthy for longer.
Like it is possible to increase the quantity of some nutrients and make them more effective it is also possible to reduce nutrients like carbohydrates from foods that are consumed a lot or make them indigestible without affecting the taste of food.