The term F1 stands for Filial 1. It is often used to describe seeds. An F1 seed is produced from two known varieties of plant. The same two varieties are always used to produce the F1 seed. The seed produced always has the same characteristics. If you grow a plant from an F1 seed the plant will always be exactly the same.
In almost all cases it is not possible for the amateur gardener to produce F1 seed. Two F1 plants cannot be used to produce the same F1 seed.