The cage is inserted between two bones. The force of the pressure of the bones against each other (and now against the cage) is enough to hold it in place. Sometimes the doctor uses a metal plate or screws to help hold everything together until it heals. Bone graft material is also used to fill in where the bone once existed.
The best place for the cage is right in the middle of the vertebra above or below it. The cage can shift or slide. A small amount of slippage is okay. Too much slippage and the fusion may fail or the spine deforms. The doctor will keep an eye on this using periodic X-rays.
You may be given a rigid, plastic corset to wear for the first six (or more) weeks. Don't throw it in a corner or closet. The corset will help stabilize the spine while it heals. It can also help take some of the load off the nearby discs and healing area.