The _SFR_BYTE() is a macro that returns a byte of data of the specified address.
The _SFR_BYTE() is defined in hardware/tools/avr/avr/include/avr/sfr_defs.h as below.
#define _SFR_BYTE(sfr) _MMIO_BYTE(_SFR_ADDR(sfr))
It returns a macro that returns a byte of data of the specified address. The _SFR_ADDR() is a macro that returns the address and the _MMIO_BYTE is a macro that returns a byte of data of the specified address.