From Gray's Anatomy - the zona
pellucida is the thick white layer.
Following conception, the blastocyst 'hatches' out of the tough shell surrounding it, called the zona pellucida. This is necessary for it to attach to the uterine wall and begin to receive oxygen and other nutrients from the mother. Sometimes, the blastocyst can’t hatch from the tough zona pellucida because it’s too tough, effectively disabling the embryo.
Assisted Zona Hatching (AZH) gives the blastocyst a helping hand, sort of. In the IVF lab, the zona pellucida is either thinned out or a hole is put into it, allowing the blastocyst to do the rest on its own. The embryo is then returned to the uterus.
AZH is applied during the 3rd step of in vitro fertilization, the fertilization step.