Apexification is a treatment used to seal an open apex. The apex is the bottom part of a tooth’s root. Normally teeth have cone shaped roots, these root canals are usually treated with conventional methods. However, if the canal is not closed at the root end of the tooth apexification is required.
An open apex is often found in teeth that are not fully developed or whose roots have been eroded away by a process called resorption. An open apex makes a root canal procedure complicated by making it difficult to seal the root canal. An apexification is a procedure that is used to form a hard barrier at the end of a root with an open apex. When it is determined that a root end barrier has formed, the root canal therapy will be completed.
Source: American Association of Endodontists