The __LPM_enhanced() is an assembler code that reads a byte of data stored in a specified address(PROGMEM area).
The __LPM_enhanced() is defined in hardware/tools/avr/avr/include/avr/pgmspace.h as below.
#define __LPM_enhanced__(addr) \ (__extension__({ \ uint16_t __addr16 = (uint16_t)(addr); \ uint8_t __result; \ __asm__ \ ( \ "lpm %0, Z" "\n\t" \ : "=r" (__result) \ : "z" (__addr16) \ ); \ __result; \ }))
The lpm, short for Load Program Memory, is a instruction that reads a byte of data from PROGMEM area. It substitue a data at addr for __result, then returns the __result.