Angiosperms are plants that bear flowers (Basically, flower is nothing but the carrier of female reproductive organs of the plant). During fertilization, there are two sperms (from pollen grains) that enter the ovary via the formation of a pollen tube. Out of these two sperms, one fuses with the female egg and forms zygote. This is first fertilization. In angiosperms, two fertilizations happen (also called double fertilization).
The other one fuses with two polar nuclei (polar nuclei are located at the poles of the sac) and forms nutrient-rich endosperm. Now, since three haploid cells fuse in the latter case, it is called as “triple” fusion. Triple fusion is significant in angiosperms because the result of this triple fusion, i.e. formation of the endosperm is very important for the developing embryo. This is because it provides the embryo with all the nutrition that it needs during the early days of its growth. Note that endosperm is triploid unlike the zygote which is haploid.