From the about page:
Invisible in most major operating systems until recently, the unicode specification defines a bunch of weird tables which are intended to be used as mathematical variables, for people who are not satisfied with good old a to z, a′ to z′, a′′ to z′′, etc.<p>Many of these are composed of the full roman alphabet, meaning a fun hack is possible which allows you to type with a weird font in places where you cannot normally use formatting – such as a filename, facebook status, or anywhere you can type unicode text.