It tells you what the paragraph is going to be about.
An F1 hybrid (or filial 1 hybrid) is the first filial generation of offspring of distinctly different parental types. F1 hybrids are used in genetics, and in selective breeding, where it may appear as F1 crossbreed. The term is sometimes written with a subscript, as F1 hybrid.
1 The main advantage of F1 hybrids in agriculture is also their drawback. 2  When F1 cultivars are used as parents, their offspring (F2 generation) vary greatly from one another. 3  Some F2s are high in homozygous genes, as found in their grandparents, and these will lack hybrid vigour.