Ventral means in front. Thecal sac means the sac that contain the spinal cord and cerebro spinal fluid. Effacement of the ventral thecal sac means pressure is upon the front of the sac (producing a flattening effect) most likely from a disc protrusion. Foramen means hole (where the nerve comes out of from the spinal cord). Stenosis means narrowing (usually from bony degeneration or disc herniation/protrusion/or bulge). Left foraminal stenosis means narrowing of the passageway where the nerve comes through.