It's basically the same as selective breeding, but instead of waiting potentially many generations for DNA to randomly combine until you get the desired traits, you get them straight away by altering the DNA to produce those traits.
It would be used to make crops more weather resistant, disease...
Actually this is one of the first things that's come out of Brexit that I think is a good outcome. We're going to need GE/GM crops to keep up with climate change, decreasing biodiversity and population growth, and despite the sci-fi movies suggesting otherwise they will be perfectly safe and...