The best way I could understand the spine is to think of your spinal cord like a tree with branches. The spinal cord that runs from your head to your "behind" is the tree trunk. Off the spinal cord there are "nerve roots", like tree branches. There are branches at every step down your vertebrae. The spine is labeled levels Cs for neck, T's for Trunk (thoracic) and Ls for Lumbar (lower back). Depending on the number that is the level where you neural foramina is narrowed.
The neural foramin are canal like space where those "tree braches" nerves roots exit the spinal canal to your extremities, body , etc.. There is one on each side (right and left) formed by the bony part of your spine.
Narrowing is most usually caused by bone spur /ridges like growth, or sometimes disc material bulging into that space. Usually causing pain, tingling, or numbness.