How does double fertilization work in angiosperms?

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The pollen grain (male gametophyte) has two cells. The tube cell makes a tube down the style of a flower down to the female gametophyte inside the flower's ovary.  The sperm cell goes down that tube. During the trip, it divides into two sperm. One sperm cell fertilizes the egg cell of the female gametophyte (forming the new plant embryo), the other fertilizes a cell with 2 nuclei, forming a cell with 3 haploid nuclei ("trikaryotic).  That develops into the endosperm, a kind of food reserve for the embryo in the seed.