"Feminized" means that the seed is guaranteed to be a female.
reason that this can happen, is that plants can "Hermie". This means
that a female plant can become a "Hermaphrodite" through stress. Because
it feels it needs to make seeds, and will not find a male plant in
time. It pollinates itself, makes seeds and these seeds are all females.
is best for most indoor growers, but can also cause problems as
feminized seeds tend to hermie easier than regular female plants, from
seeds pollinated by a male plant.
There is another problem with feminized seeds.
is no potential for breeding. You will get no male plants in a bag of
feminized seeds, and therefore cannot breed in any way, "cross breed" in
any way, or make a new strain of your own, or anything.
You can just grow buds, and harvest them.
But that's all a lot of people are trying to do.