In order for the embryo to implant in the uterine endometrium, it must break and leave the covering (zona pellucida) that surrounds it. The hatching is what is known as the rupture of this membrane to allow the exit of embryonic cells and implantation.
Therefore, assisted hatching is nothing more than making a small hole in the zona pellucida of the fertilized egg, or embryo, to facilitate implantation.
In some IVF-ICSI situations, it is convenient to make hatching to facilitate implantation. One situation in which it would be beneficial to make this small hole is in those embryos with a thickened zona pellucida.