A blastocyst is a highly developed embryo that is nearly ready to implant on the walls of the uterus.
During maturation an embryo rests inside a protective shell. You can think of this protective shell as being much like a chicken egg. The embryo hatches on the fifth or sixth day and implants itself to the uterine wall. Just prior to hatching, this embryo becomes a blastocyst.
Embryos developing to the critical blastocyst stage have a much greater chance of implanting successfully and resulting in an ongoing pregnancy. During the first few days, the embryo relies on the mother's egg for nutrition. However, in order to survive past the first three-four days, the embryo must become a blastocyst. Only about one-third of the embryos are successful. These embryos are more highly-developed, healthier, and stronger, and have a higher rate of implantation when compared to day three embryos.