The pgm_read_byte() is a macro that reads a byte of data stored in a specified address(PROGMEM area).
The pgm_read_byte() is defined in hardware/tools/avr/avr/include/avr/pgmspace.h as below.
#define pgm_read_byte(address_short) pgm_read_byte_near(address_short)
The input is address_short and calls the pgm_read_byte_near() macro.