An IBL (inbred line) is a genetically homogeneous strain that grows uniformly from seed.
A hybrid is a strain made up of two genetically unlike parents, IBL or hybrid.
When you cross two different IBL strains for the FIRST time, it is called the F1 generation. When you cross two of the same F1 hybrid (inbreed), it is called the F2 generation.
The process of selective inbreeding must continue at least until the F4 to stabilize the recurrently selected traits. When you cross two specimens of an IBL variety, you get more of the same, because an IBL is homozygous, or true breeding for particular traits.