If your missing tooth is at the back of the upper jaw, there's an extra factor to consider: the maxillary sinus, an air-filled space that sits just above the roots of your upper back teeth. In some people, especially after a tooth's been missing for a long time, the sinus floor can sit quite close to the available bone, leaving too little height for an implant.
A sinus lift addresses this directly — the sinus membrane is gently lifted, and graft material is placed underneath it to build up the bone height needed. It's a well-established, specific type of bone grafting used only in this part of the jaw, and it's only ever recommended if your imaging shows it's genuinely required for your implant to have a stable foundation.