The longitudnal fissure is a deep groove along the longitudnal portion of the brain, dividing it into hemispheres. The corpus callosum helps connect these two portions which are divided by the longitudnal fissure with millions of axons of interneurons to relay information between these two halves.

Imagine a line continuing up the way the arrow points all the way to the top of the picture. This is the line creating the longitudnal fissure of the brain.