Dear student,
The endosperm in angiosperm is formed when one of the sperm nuclei fuses with the polar nuclei of the embryonic sac during pollination, leading to production of a triploid (3n) endosperm, inside the seed. Therefore,in angiosperms it is called secondary endosperm. Thus, the endosperm forms the tissue within the seed which has the function of nourishing the embryo.
Regards.