what's the meaning of "%%fs" ?

hi all: the codes is: extern inline unsigned char get_fs_byte(const char * addr) { unsigned register char _v; __asm__ ("movb %%fs:%1,%0":"=r" (_v):"m" (*addr)); return _v; } What's the meaning of "%%fs" ? Is " __asm__ ("movb %1,%0":"=r" (_v):"m" (*addr));" not ok? Thank you.