The _MMIO_WORD() is a macro that returns two bytes of data at the specified address.

The _MMIO_WORD() is defined in hardware/tools/avr/avr/include/avr/sfr_defs.h as below.

#define _MMIO_WORD(mem_addr) (*(volatile uint16_t *)(mem_addr))

The input is mem_addr and returns its contents.