What is meant by embryo sac?

The embryo sac is the female gametophyte of angiosperms, a microscopic haploid structure inside the ovule that houses the egg and enables double fertilization and seed formation. 

The typical mature sac has eight nuclei in seven cells: one egg cell flanked by two synergids, a central cell with two polar nuclei, and three antipodal cells at the chalazal end. It develops from a single functional megaspore via megagametogenesis after meiosis of the megaspore mother cell. 

During fertilization, one sperm fuses with the egg to form the zygote, and another fuses with the polar nuclei to form the endosperm; the synergids aid pollen tube entry, while antipodals have no confirmed reproductive role.