An Apico - technically an apicoectomoy - is a type of endodontic microsurgery, using specialized tools and a microscope.
When an Apico is performed, a small opening is made at the the tip of the root in order to remove an infected tissue and the root tip itself. Then, a small filling is placed to seal the end of the root.
Why would you need an Apico? If a canal is blocked or innacessible, or if you have a fracture or crack in your tooth's roots, it can cause a root canal to fail. An Apico can help in these situations.
For more information about apicos from the American Association of Endodontics (AAE), Click Here.