What is self incompatibility

Explanation :- 

Self - incompatibility is the natural inability of the plant in which pollen grains are not able to germinate on the stigma of the same flower.

In self - incompatible species, pollen grains do not lead to seed formation because pollen grains are not able to germinate on the stigma and hence the process of fertilization is not carried out to form embryo and later on seeds in such flowers.