What is the P1 generation? What did Mendel mean when he said true breeding?

The P1 generation is the first generation that the mother and father plants created (the offspring of the initial plants).

True blending is where the genes from the mother and father blend together to create a blend of their genes.

Explanation:

Blending - for example, if a purple-flowered plant is bred with a white-flowered plant, then true blending would suggest that the offspring would have pink flowers (a blend of purple and white)