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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.
You will see many hybrid tomatoes labeled F1, which means it is a new tomato variety, or first generation. A tomato labeled F2 simply means it is the second generation of that tomato variety. Unlike heirlooms, seeds collected from a hybrid will not grow identical to the parent. It will grow with the traits of one of the parent plants, but not both. Many hybrid seeds are sterile and will not even germinate.