We’ve been breeding various strains of plants and animals for a very long time. But GM food is different. They’re inserting genes that were not there originally rather than selecting genes that are already present.
What Monsanto is doing is not the same as what Gregor Mendel and Luther Burbank did.
Monsanto is not selecting naturally occurring genetic variants of a plant a specific traits (e.g. natural higher yield), and specifically breeding that variant or even cross breeding it with other strains of the same plant with different traits.
Monsanto is inserting a gene for a desired trait from one plant into another, e.g. if there is a strain of peanuts that are resistant to a mold that affects a strain of corn, the peanut gene responsible for the resistance is inserted into the corn gene. GMO crops are artificially modified genetic organisms.