A bridge fills, or 'bridges' a gap left by a missing tooth or teeth.
A gap most often occurs because advanced decay has made it necessary to extract a tooth, or perhaps because a tooth has been knocked out in an accident. Leaving the gap open can put a strain on the teeth either side. It can also affect your 'bite' - the teeth on either side of the gap may lean together into the space and alter the way that the upper and lower teeth meet; in turn causing a number of other problems. A bridge is one way of filling the gap to improve your appearance.
Bridges should last for many years. However, the life expectancy of any bridge is greatly influenced by factors under your own control, such as diet and how well you look after your teeth.