According to Mendel for each gene in a pea plant there are two alelles. These two alles are what decide the phenotype or physical properties of a plant. For example Pea Plants can either be tall or dwarf. The alelle to produce a dwarf plant is recessive to the dominant tall alelle. A true breeding plant is one which has identical allelles for a particular characteristic.
Allowed a plant or an animal to produce there own kind, ie (producing offspring that resemble there perent for generation.