here’s what apple says about font validation:
Font validation
Mac OS X Tiger makes it much easier to identify fonts that may cause problems in
displaying or printing documents when you use them with your applications and the
operating system. By default, Font Book validates a font when it is installed. Each font
file is composed of a series of attributes that the system needs to display and print
the font. To determine whether a font is valid, it checks the font files to make sure that
these attributes are correctly formatted. The following are some of the attributes that
Font Book checks when validating a font.
Font Table Directory: Table directory of the OpenType and TrueType font formats.
Glyph Location Table: OpenType and TrueType data table storing the offsets to the locations
of the glyphs in the font.
Horizontal Metrics: Table OpenType and TrueType data table containing the advance width and
left-side bearing of the font.
PostScript Compatibility Table: OpenType and TrueType data table containing additional information
needed to use the font on PostScript printers, including the FontInfo
dictionary entry and PostScript names of all the glyphs.
Character Code Mapping / Table Directory: OpenType, TrueType, or synthesized data table defining the mapping
of character codes to the glyph index values used in the font; it may
contain more than one subtable, each supporting a different character
encoding scheme.
Intercharacter Spacing Table: TrueType data table containing the values that control the intercharacter
spacing for the glyphs in a font.
Font Name Table: OpenType, TrueType, or synthesized data table containing strings
associated with the font, including copyright notices, font names,
family names, and style names.
After Font Book validates the font, a pane displays the results. Depending on what is
found, you will see one of the following for each font:
• Passed. The font is safe to use and can be installed without problems.
• Minor problems. This result means only that duplicate fonts have been found. Because
Mac OS X handles duplicate fonts without any problems, this alert simply indicates that
you may want to remove one of the fonts.
• Serious problems. You should not install this font. Contact the original font vendor
to obtain a properly formatted font. If you need to send an error report to the vendor,
you can create one by selecting Save Report from the File menu.
If the font passes validation, you can check the box next to the font to enable the
Install button and continue the installation.