The _LPM_word_enhanced_() is an assembler code that reads a word of data stored in a specified address(PROGMEM area).
The _LPM_word_enhanced_() is defined in hardware/tools/avr/avr/include/avr/pgmspace.h as below.
The lpm, short for Load Program Memory, is a instruction that reads a byte of data from PROGMEM area. In this macro, the lpm instruction is called twice to get two bytes of data. It substitue a data at addr for __result, then returns the __result.
Arduino 1.8.13/avr-gcc 7.3.0-atmel3.6.1-arduino
June 19, 2020