3.4.5 Unicode Strings
The special operators __utf16__, __utf16le__, __utf16be__, __utf32__, __utf32le__ and __utf32be__ allows definition of Unicode strings. They take a string in UTF-8 format and converts it to UTF-16 or UTF-32, respectively. Unless the be forms are specified, the output is littleendian.
For example:
%define u(x) __utf16__(x)
%define w(x) __utf32__(x)
dw u('C:\WINDOWS'), 0 ; Pathname in UTF-16
dd w(`A + B = \u206a`), 0 ; String in UTF-32
The UTF operators can be applied either to strings passed to the DB family instructions, or to character constants in an expression context.
from http://www.nasm.us/doc/nasmdoc3.html