The _MMIO_BYTE() is a macro that returns a byte of data at the specified address.

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

#define _MMIO_BYTE(mem_addr) (*(volatile uint8_t *)(mem_addr))

The input is mem_addr and returns its contents.