The intervertebral foramen is an opening made by two adjacent, stacked vertebrae. In the spinal column, stacking occurs down the front part of the bones (called the bodies).
Behind the body of a vertebra is a ring of bone. Looking at a vertebra from either side, the body and ring combination make an arch-like shape. A hole is created by the meeting of the "pillars" of the arch with the bone below. This hole is called the intervertebral foramen.
The nerve root exits out the intervertebral foramen after branching off the spinal cord.