Nothing to do with GM - it is simply conventional plant breeding.
F1 means the "first filial generation" seeds/plants - a hybrid.
Essentially, the seed producer has two distinct parents and cross-pollinates those two varieties to produce a hybrid with desirable characteristics. This is often done by hand, which is why the seed is more expensive. F1 seed often gives better crops/nicer flowers and that is why this is done.
Some people on here argue that F1 seed is a bad thing, because if you save the seed yourself, they won't come true to the parents and because the lack of genetic diversity may not be so good for pest/disease resistance.
Also, the 'desirable characteristics' may be desirable for the farmer, rather than gardener - for example if the whole crop ripens at the same time, that is good for mechanical harvesting, but not good for someone who wants to pick a few of something each day.
