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.
F1 is a breeding of a golden retriever and a poodle. The standard doodle would be with a standard size poodle and the mini would be with a mini size poodle. They have the highest possibility of hybrid vigor and a reduced risk of genetic defects. F1B is a breeding of an F1 goldendoodle with a poodle. This means 75% poodle and 25% golden retriever.