What is a polygon island in Altium?
So what exactly is a polygon island in Altium


This is explained in the online documentation:


  
    Remove Islands Less Than
    
    Remove any areas of copper that are less than the specified area. This option is area-based only; it does not consider if that island is connected to a pad or via. Note that all small unconnected areas of copper are always removed if the Remove Dead Copper option is enabled in the Properties panel.
  


It's a small part of the polygon that is unconnected to the rest of the polygon due to other objects like tracks or vias.


  How can they be removed from the PCB?


By checking the box for "remove islands less than" and choosing the maximum island area in the polygon properties dialog.