The Last Resort font is a collection of glyphs to represent types of Unicode characters. These glyphs are designed to allow users to recognize that an encoded value is one of the following:

   a specific type of Unicode character
   in the Private Use Area (no private agreement exists)
   unassigned (reserved for future assignment)
   one of the illegal character codes.

These glyphs are used as the backup of "last resort" to any other font; if the font cannot represent any particular Unicode character, the appropriate "missing" glyph from the Last Resort font is used instead. This provides users with the ability to tell what sort of character it is, and gives them a clue as to what type of font they would need to display the characters correctly. (For more information, see The Unicode Standard, Section 5.3 Unknown and Missing Characters.)
Ironically, it is under export control.