what is endosperm? how is it formed ? why is it absent in some seeds ?

In flowering plants, as you know, 2 male gametes enter the egg apparatus during fertilization (known as double fertilization in flowering plants). One of the male gametes fuses with the egg cell resulting in the formation of a diploid cell, the zygote.

 The other male gamete fuses with the two polar nuclei located in the central cell to produce a triploid primary endosperm nucleus (PEN) and the central cell becomes the primary endosperm cell (PEC). PEC later develops into the endosperm. You can observe the diagram below

This endosperm formed provides nutrition to the growing embryo usually in the form of starch but may also contain certain proteins, oils etc.

In some seeds known as non endospermic seeds, the endosperm is absent at maturity since it is completely consumed by the growing embryo. E.g, in pea and beans.