True breeding = Always producing offspring with the same traits as the parents when the parents are self-fertilized.   Mendel started his experiments with true-breeding plant varieties, which he hybridized (cross-pollinated) in experimental crosses.  
· The true-breeding parental plants of such a cross are called the P generation (parental).

· The hybrid offspring of the P generation are the F1 generation (first filial).

· Allowing F1 generation plants to self-pollinate produces the next generation, the F2 generation (second filial).