Genetically modified plants are ones with desired characteristics which are meant to enhance a crop.
They may have DNA which gives them more nutritional value: like golden rice which is rice with carotene in. This will benefit a population which eats a lot of rice by giving them a better diet.
Plants may also have DNA which makes them more resistant: for example genetically modified soya plants. This means that a broad spectrum herbicide that kills many different types of plant can be spread on the crops to kill weeds; where as before many different types of herbicides would have to be spread to avoid killing the crop (time consuming/ expensive.)
Although GM plants appear to be only of benefit, there are people who claim side effects observed in lab animals such as: sterility; infant mortality; allergies; stunted growth. Another disadvantage would be that a broad spectrum herbicide that can be spread on GM crops will kill many plants in the wild not just the weeds threatening the crops.