A dental implant is a bio-compatible (meaning your body likes it), man-made substitute to replace missing tooth roots; it is usually made of a space-age alloy mostly of titanium. It's the same material now being used in other joints such as hips and shoulders.
Implants come in various shapes and sizes to fit the situation. In fact, new versions are constantly being made available so that even if you were previously told implants weren't for you there is likely a new version that will work for you.
Most are cylinders placed into bone and allowed to heal undisturbed while bone heals around them, locking them in like an anchor. After a few months, the implants are used as a foundation for replacing the missing teeth. For some patients with excellent bone, the implant can be restored right away.