It sets or gets the kerning factor on a character-level basis, where a factor of 0.0 means no kerning at all, 1.0 uses the kerning specified by the font designer, and any intermediate value between 0.0 and 1.0 uses a loosened kerning somewhere between no kerning and the default kerning. Of course, the kerning factor has no effect on fonts that don't have kerning tables.