They are histological evidence of the small amount of cytoplasm that remains in the inner layer of the myelin sheath created by Schwann cells wrapping tightly around an axon (nerve fiber).

These layers are generally uniform and continuous, but due to imperfect nature of the process by which Schwann cells wrap the nerve axon, this wrapping process can sometimes leave behind small pockets of residual cytoplasm displaced to the periphery during the formation of the myelin sheath.

These pockets, or "incisures", can subdivide the myelinated axon into irregular portions.

Secondary incisures appear later, in regions of a compact myelin sheath, initially traversing only part of its radial thickness but commonly occupying its whole circumference.

[1] This article incorporates text in the public domain from page 727 of the 20th edition of Gray's Anatomy (1918)