Spinal fusion. Transforaminal lumbar interbody fusion means a "cage" prosthesis and bone graft material is placed in the front of the spine through an approach through the back to join 2 bones. It is performed through the area where the nerve usually exits off the main spinal sac (called the neuro foramen). This foramen is an opening that allows access to the front of the spine through an incision on the back.
Just another way of . Doing a fusion by placing a bone graft usually put in an implant into the cleared out disc space (interbody or between the vertebral bodies where the disc is found) prepared through a more lateral or to the side approach around the nerve root followed by spinal instrumentation like a rod and screw system. That lateral approach is thru the foramen or nerve hole exit, hence, transforaminal.