A germ layer is a collection of cells, formed during animal embryogenesis.
Germ layers are only really pronounced in the vertebrates.
However, all animals more complex than sponges (eumetazoans and agnotozoans) produce two or three primary tissue layers (sometimes called primary germ layers).
Animals with radial symmetry, like cnidarians, produce two called ectoderm and endoderm, making them diploblastic.
Animals with bilateral symmetry produce a third layer in-between called mesoderm, making them triploblastic.
Germ layers will eventually give rise to all of an animal’s tissues and organs through a process called organogenesis.