The __LPM() is a macro that reads a word of data stored in a specified address(PROGMEM area).

The __LPM_word() is defined in hardware/tools/avr/avr/include/avr/pgmspace.h as below.

#define __LPM_word(addr)    __LPM_word_enhanced__(addr)

The input is addr and just calls __LPM_word_enhanced__().