these are characters that modify the following character when a string is rendered. Typically these are accent marks, though some Mid-Eastern languages have symbols that can seriously change how the next character is rendered.
These make 100% correct editing and drawing of Unicode extremely difficult. However you can easily make almost everybody happy by detecting the few that have matching "combined" equivalent characters (such as á which is a unicode character that can replace the combining acute accent and the a), and otherwise ignoring these or treating them as individual characters. Don't refuse to draw stuff just because you encounter these, that is worse than drawing it wrong.