Every living organism has its own unique DNA and genes that makes it what it is, for example, visual characteristics or habits. Genes are inherited from your parents or are given to you by your parents. the science behind GM foods is that scientists take certain characteristic genes from one food, and then insert it into another food, to make the second food grow according to that gene, therefore genetically modifying it!
For example, if a scientist wanted to grow a kiwi that looked like an orange from the outside, they would take a gene from an orange that made it grow an orange skin that you can peel, then insert it into a growing kiwi plant, which makes all its fruit grow orange skin, therefore creating a hybrid fruit, which I, personally, would name a kirange!