This refers to a tooth or teeth (primary or permanent) that have become "fused" to the bone, preventing it or them from moving "down" with the bone as the jaws grow. This process can affect any teeth in the mouth, but it is more common on primary first molars and teeth that have suffered trauma (typically the incisors). Treatment can vary depending on the degree of severity of the ankylosis (how "sunken into the gums" a tooth may appear).
The degree of severity usually will vary depending on how early the process started, and as a general rule, the earlier it starts, the more severe the ankylosis becomes with age. Several considerations must be taken before any treatment is provided, and your dentist will discuss all the risks and benefits of each treatment option.