Ok, great. Now if only I knew what that meant.

See
 http://xml.apache.org/fop/faq.html#pdf-characters
To elaborate: a glyph is what is shown on the monitor or the
paper. A font basically provides a mapping from character codes
to glyphs. If the font does not provide such a mapping for a
certain character code, you are out of luck.
You can use some other utility, for example the Windows font
viewer, to confirm whether your font provides a glyph for a
particular character or not.

Am I supposed to add something to my command line or change my fo
coding? (Sorry, I just started using the FOP engine.)

Did you read
 http://xml.apache.org/fop/fonts.html