I believe the best way to explain is that three times as many tall plants as short plants. The parental generation is the first set of parents crossed, the F1 generation consists of all the offspring from the parents, while the F2 generation consists of the offspring from allowing the F1 individuals to interbreed. From the Mendel's experiment the F2 generation results from self-pollination of F1 plants, and contained 75% of tall plants and 25% of short plants.