The _BV() is a macro that shifts 1 to left by the specified numnber.
The _BV() is defined in hardware/tools/avr/avr/include/avr/sfr_defs.h, as shown below.
#define _BV(bit) (1 << (bit))
The _BV() is a macro that shifts 1 to left by the specified numnber. For example _BV(5) would be 0b00100000.