A blastocyst is an early stage of embryonic development that occurs about five to six days after fertilization. It consists of a hollow ball of cells, with an inner cell mass that will eventually develop into the embryo and an outer layer called the trophoblast, which will form part of the placenta. During this stage, the blastocyst implants itself into the uterine wall, allowing for further growth and development. This process is crucial for establishing a successful pregnancy, as it enables the embryo to receive nutrients and support from the mother's body.