What is a gmo

As we all know in English we have to write a 7 page essay on food and my question that I am really focusing in on is What is Gmos really? We see in the store it says that this product does not contain Gmos. But us as the people we are we just be like oh okay but who really know what has Gmos In it they never tell us oh this product contains _% of Gmos and that really makes me angry I just don't know why would anyone try to even Dj this