The blastopore is the opening into the gastrulated embryo, and the archenteron is the cavity inside, which develops into the alimentary canal or digestive system.
Animal embryos begin as a single fertilized egg cell (zygote) which then divides to produce multiple cells of the same type. After the solid ball of cells (morula) gets to a certain size (at least 128 cells), the interior cells begin to migrate toward the outside, eventually forming a hollow ball called a blastula.
As it continues to grow, one pole of the blastula begins to push inward, forming an infolding or invagination in a process called gastrulation. Eventually, the gastrula forms two or three different layers of cells, producing different germ layers.
The germ layers will develop into different types of tissues as the animal develops.
Therefore, the blastopore is the opening into the invagination, and the archenteron is the cavity that eventually becomes the alimentary canal.
Image: Copyright Pearson Education, Inc., publishing as Benjamin Cummings.